Segment Reporting (Schedule of Information for Reportable Segments) (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Segment Reporting Information [Line Items]                        
Rental revenue $ 46,467 $ 46,569 $ 45,518 $ 43,950 $ 41,294 $ 37,266 $ 36,205 $ 27,602 $ 24,022 $ 177,332 $ 125,095 $ 56,431
Rental and parking expenses (9,128)       (8,290)         (37,327) (26,096) (8,164)
Income from operations 14,196 15,942 16,655 15,394 15,245 13,340 12,225 9,714 8,555 63,237 43,834 15,687
General and administrative expenses (1,403)       (943)         (5,396) (4,069) (3,105)
Acquisition related expenses                   0 0 (5,339)
Asset management fees (3,494)       (3,099)         (13,114) (9,963) (4,925)
Depreciation and amortization (18,246)       (13,717)         (58,258) (41,133) (19,211)
Interest and other expense, net (9,835) (9,476) (8,938) (8,208) (7,741) (6,932) (6,786) (5,073) (3,764) (34,364) (22,555) (4,390)
Net income attributable to common stockholders 4,361 $ 6,466 $ 7,717 $ 7,186 7,504 $ 6,408 $ 5,439 $ 4,641 $ 4,791 28,873 21,279 11,297
Operating Segments [Member]                        
Segment Reporting Information [Line Items]                        
Rental revenue 46,467       41,294         177,332 125,095 56,431
Rental and parking expenses (9,128)       (8,290)         (37,327) (26,096) (8,164)
Income from operations 37,339       33,004         140,005 98,999 48,267
Operating Segments [Member] | Data Centers [Member]                        
Segment Reporting Information [Line Items]                        
Rental revenue 26,677       23,721         103,226 62,377 12,929
Rental and parking expenses (6,965)       (5,937)         (27,289) (17,571) (2,509)
Income from operations 19,712       17,784         75,937 44,806 10,420
Operating Segments [Member] | Healthcare [Member]                        
Segment Reporting Information [Line Items]                        
Rental revenue 19,790       17,573         74,106 62,718 43,502
Rental and parking expenses (2,163)       (2,353)         (10,038) (8,525) (5,655)
Income from operations $ 17,627       $ 15,220         $ 64,068 $ 54,193 $ 37,847