Investments - Additional Information (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2018
USD ($)
borrower
State
Property
note
loan
Dec. 31, 2017
USD ($)
borrower
Property
note
loan
Dec. 31, 2016
USD ($)
Property
Investment [Line Items]      
Financing Receivable, Number of Financing Receivables, Secured | note   1  
Real estate properties and loans, gross $ 5,123,631 $ 7,903,025 $ 8,247,654
Number of real estate properties invested in | Property 1,514 2,480 2,615
Number of states properties dispersed geographically | State 49    
Number of states exceeding disclosure threshold | State 1    
Minimum percentage of investment in real estate properties 10.00%    
Number of notes receivable | note 2 3  
Number of other notes receivable, non-secured | note   2  
Amortization amount to rental revenue for capitalized leases $ 5,500 $ 6,500 $ 6,600
Leases amortization expenses 32,600 43,300 46,400
Below market lease acquired $ 400    
Real estate assets held-for-sale expected selling period 12 months    
Impairment of properties classified as held for sale $ 1,000 24,800 20,200
Impairment of properties classified as held and used $ 16,700 $ 77,200 68,100
In-place Leases      
Investment [Line Items]      
Lease intangibles weighted average amortization period 14 years 13 years 9 months 18 days  
Acquired finite-lived intangible assets $ 21,700    
Above market leases      
Investment [Line Items]      
Lease intangibles weighted average amortization period 10 years 1 month 6 days 9 years 6 months  
Acquired finite-lived intangible assets $ 3,500    
Below-market Leases      
Investment [Line Items]      
Lease intangibles weighted average amortization period 17 years 8 months 12 days 17 years 6 months  
Leases      
Investment [Line Items]      
Lease intangibles weighted average amortization period 10 years 7 months 6 days 10 years 7 months 6 days  
Single-Tenant Commercial Properties | First Mortgage      
Investment [Line Items]      
Number of first-priority mortgage loans | loan 3 10  
Number of borrowers | borrower 3 6  
Tenant Assets and Stock      
Investment [Line Items]      
Financing Receivable, Number of Financing Receivables, Secured | note 1    
Other notes receivable $ 100 $ 3,500  
Texas      
Investment [Line Items]      
Percentage of investment in real estate properties 12.00%    
Financed Properties      
Investment [Line Items]      
Real estate properties and loans, gross $ 47,044 $ 79,967 $ 66,578
Number of real estate properties invested in | Property 52 88 74