Investments - Narrative (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2016
USD ($)
State
Property
loan
Sep. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
State
Property
loan
renewal_option
Sep. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Property
loan
Investment [Line Items]          
Gross investment in real estate properties and loans $ 8,388,494,000   $ 8,388,494,000   $ 8,302,688,000
Number of real estate properties | Property 2,705   2,705   2,629
Number of properties securing mortgage loans | Property 75   75    
Portfolio disbursement, number of states | State 49   49    
Minimum percentage of investment in real estate properties 10.00%   10.00%    
Number of other notes receivable | loan     2    
Allowance for loan losses $ 0   $ 0   $ 300,000
Amortization amount to rental revenue for capitalized leases 1,800,000 $ 1,500,000 4,700,000 $ 4,300,000  
In-Place Leases          
Investment [Line Items]          
Leases amortization expenses 11,600,000 $ 12,400,000 35,100,000 $ 37,800,000  
Secured Debt          
Investment [Line Items]          
Notes receivable $ 3,800,000   $ 3,800,000    
Minimum          
Investment [Line Items]          
Number of lease renewal options | renewal_option     1    
Texas          
Investment [Line Items]          
Number of states exceeding disclosure threshold | State     1    
Percentage of investment in real estate properties 12.30%   12.30%    
Mortgages          
Investment [Line Items]          
Receivables on nonaccrual status | loan 0   0   0
Notes Receivable          
Investment [Line Items]          
Allowance for loan losses $ 0   $ 0   $ 324,000
Receivables on nonaccrual status | loan 0   0   1
Loans on non-accrual status         $ 300,000