Summary of Significant Accounting Policies (Narrative) (Details)
 3 Months Ended 12 Months Ended
Jan. 02, 2019
USD ($)
Dec. 31, 2018
USD ($)
metropolitan
lease
class
tenant
$ / shares
shares
Mar. 31, 2019
USD ($)
metropolitan
lease
segment
tenant
intangible_asset
micropolitan
$ / shares
shares
Dec. 31, 2018
USD ($)
metropolitan
lease
class
tenant
$ / shares
shares
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2018
USD ($)
metropolitan
lease
class
segment
tenant
$ / shares
shares
Dec. 31, 2017
USD ($)
segment
$ / shares
shares
Dec. 31, 2016
USD ($)
shares
Jan. 01, 2019
Dec. 31, 2015
shares
Summary of Significant Accounting Policies [Line Items]                                
Rental revenue     $ 46,467,000 $ 46,569,000 $ 45,518,000 $ 43,950,000 $ 41,294,000 $ 37,266,000 $ 36,205,000 $ 27,602,000 $ 24,022,000 $ 177,332,000 $ 125,095,000 $ 56,431,000    
Impairment losses on real estate and related intangible assets   $ 0                            
Allowance for uncollectible tenant receivables   $ 0   $ 0               $ 0        
Diluted earnings per share outstanding adjustment (in shares) | shares     26,000       18,000         24,000 18,000 16,000    
Number of reportable business segments | segment     2                 2 2      
Number of metropolitan statistical areas with owned real estate investments | metropolitan   42 42 42               42        
Number of micropolitan statistical areas with owned real estate investments | micropolitan     1                          
Shares authorized | shares   600,000,000   600,000,000               600,000,000        
Common stock, shares authorized (in shares) | shares   500,000,000 500,000,000 500,000,000       500,000,000       500,000,000 500,000,000      
Common stock, par value (in dollars per share) | $ / shares   $ 0.01 $ 0.01 $ 0.01       $ 0.01       $ 0.01 $ 0.01      
Preferred stock, shares authorized (in shares) | shares   100,000,000 100,000,000 100,000,000       100,000,000       100,000,000 100,000,000      
Preferred stock, par value (in dollars per share) | $ / shares   $ 0.01 $ 0.01 $ 0.01       $ 0.01       $ 0.01 $ 0.01      
Number of classes of common stock | class   4   4               4        
Common stock, shares issued (in shares) | shares   143,412,353 144,534,765 143,412,353       126,559,834       143,412,353 126,559,834      
Common stock, shares outstanding (in shares) | shares   136,466,242 136,428,375 136,466,242       124,327,777       136,466,242 124,327,777      
Preferred stock, shares issued (in shares) | shares   0 0 0       0       0 0      
Preferred stock, shares outstanding (in shares) | shares   0 0 0       0       0 0      
Minimum number of classes or series of preferred stock the board of directors can issue without stockholder approval | class                       1        
Maximum number of shares available for repurchase during any calendar year, as percentage of common stock outstanding at end of prior year     5.00%                 5.00%        
Period of notice required for changes to share repurchase program     30 days                 30 days        
Repurchase of common stock, percentage                       3.80% 2.27%      
Repurchase of common stock     $ 10,733,000       $ 8,420,000         $ 43,230,000 $ 17,159,000 $ 3,114,000    
Distributions declared per common share (in dollars per share) | $ / shares     $ 0.16       $ 0.15         $ 0.63 $ 0.62      
Distributions payable   $ 7,317,000 $ 7,315,000 $ 7,317,000       $ 6,566,000       $ 7,317,000 $ 6,566,000      
Distributions paid in cash     10,813,000       $ 9,333,000         40,296,000 28,994,000 17,659,000    
Common stock issued through distribution reinvestment plan     $ 10,385,000       9,920,000         40,938,000 32,264,000 22,889,000    
Impact related to uncertain tax positions from the results of operations                       $ 0 0 0    
Parking revenue (% of total revenue)   1.43%   1.43%               1.43%        
Number of operating ground leases | lease   3 6 3               3        
Capitalization of indirect leasing costs (less than) (% of total assets)   1.00%   1.00%               1.00%        
Provision for doubtful accounts related to rental revenue     $ 445,000                          
Number of operating ground leases without corresponding operating lease liabilities | lease     3                          
Rental and parking expenses     $ 9,128,000       $ 8,290,000         $ 37,327,000 $ 26,096,000 $ 8,164,000    
In-place leases [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Number of impaired intangible assets | intangible_asset     1                          
Impairment of intangible assets     $ 2,658,000                          
Accounting Standards Update 2018-20 [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Rental revenue     (406,000)                          
Rental and parking expenses     $ (406,000)                          
Accounting Standards Update 2016-02 [Member] | Scenario, Forecast [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Operating lease, liability (less than) (% of total liabilities)                             1.00%  
Operating lease, right-of-use asset (less than) (% of total assets)                             1.00%  
December 1, 2018 To December 31, 2018 [Member] | Subsequent Event [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Distributions paid in cash $ 3,715,000                              
Common stock issued through distribution reinvestment plan 3,602,000                              
Common Stock [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Common stock, shares outstanding (in shares) | shares   136,466,242 136,428,375 136,466,242     128,021,413 124,327,777       136,466,242 124,327,777 82,744,288   48,457,191
Repurchase of common stock (in shares) | shares     1,160,279       917,212         4,700,554 1,880,820 333,194    
Repurchase of common stock     $ 12,000       $ 9,000         $ 47,000 $ 18,000 $ 3,000    
Common stock issued through distribution reinvestment plan     $ 12,000       $ 11,000         $ 44,000 $ 35,000 $ 24,000    
Class A shares [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Common stock, shares authorized (in shares) | shares   175,000,000   175,000,000               175,000,000        
Common stock, par value (in dollars per share) | $ / shares     $ 0.01                          
Class A shares [Member] | December 1, 2018 To December 31, 2018 [Member] | Subsequent Event [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Distributions paid in cash 2,432,000                              
Common stock issued through distribution reinvestment plan 2,181,000                              
Class A shares [Member] | Common Stock [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Repurchase of common stock (in shares) | shares     858,080       842,952         4,117,566 1,793,424      
Class I shares [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Common stock, shares authorized (in shares) | shares   75,000,000   75,000,000               75,000,000        
Class I shares [Member] | December 1, 2018 To December 31, 2018 [Member] | Subsequent Event [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Distributions paid in cash 404,000                              
Common stock issued through distribution reinvestment plan 294,000                              
Class I shares [Member] | Common Stock [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Repurchase of common stock (in shares) | shares     108,765                 71,180 5,457      
Class T shares [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Common stock, shares authorized (in shares) | shares   175,000,000   175,000,000               175,000,000        
Class T shares [Member] | December 1, 2018 To December 31, 2018 [Member] | Subsequent Event [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Distributions paid in cash 810,000                              
Common stock issued through distribution reinvestment plan 1,032,000                              
Class T shares [Member] | Common Stock [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Repurchase of common stock (in shares) | shares     193,434       74,260         511,808 81,939      
Class T2 shares [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Common stock, shares authorized (in shares) | shares   75,000,000   75,000,000               75,000,000        
Class T2 shares [Member] | December 1, 2018 To December 31, 2018 [Member] | Subsequent Event [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Distributions paid in cash 69,000                              
Common stock issued through distribution reinvestment plan $ 95,000                              
Class A, I, T and T2 shares [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Common stock, par value (in dollars per share) | $ / shares   $ 0.01   $ 0.01               $ 0.01        
Class A, I and T shares [Member] | Common Stock [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Repurchase of common stock (in shares) | shares     1,160,279                 4,700,554 1,880,820      
Repurchase of common stock     $ 10,733,000                 $ 43,230,000 $ 17,159,000      
Repurchase of common stock, average price per share (in dollars per share) | $ / shares     $ 9.25                 $ 9.20 $ 9.12      
Class A and T shares [Member] | Common Stock [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Repurchase of common stock (in shares) | shares             917,212                  
Repurchase of common stock             $ 8,420,000                  
Repurchase of common stock, average price per share (in dollars per share) | $ / shares             $ 9.18                  
Revenue [Member] | Geographic Concentration Risk [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Number of metropolitan statistical areas with owned real estate investments | metropolitan   2 1 2               2        
Revenue [Member] | Geographic Concentration Risk [Member] | Atlanta-Sandy Springs-Roswell, Georgia MSA [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Concentration risk, percentage     15.60%                 16.80%        
Revenue [Member] | Geographic Concentration Risk [Member] | Houston-The Woodlands-Sugar Land, Texas MSA [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Concentration risk, percentage                       10.00%        
Revenue [Member] | Customer Concentration Risk [Member]                                
Summary of Significant Accounting Policies [Line Items]                                
Number of tenants | tenant   0 0 0               0