Investments - Narrative (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2015
USD ($)
State
Property
loan
Jun. 30, 2014
USD ($)
Jun. 30, 2015
USD ($)
State
Property
loan
Jun. 30, 2014
USD ($)
Dec. 31, 2014
USD ($)
Property
loan
Investment [Line Items]          
Gross investment in real estate properties $ 8,207,792,000   $ 8,207,792,000   $ 8,044,363,000
Number of real estate properties | Property 2,600   2,600   2,509
Number of properties securing borrowings | Property 145   145    
Properties dispersed geographically | 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   $ 0
Amortization amount to rental revenue for capitalized leases 1,400,000 $ 1,700,000 2,800,000 $ 3,000,000  
In-Place Leases          
Investment [Line Items]          
Leases amortization expenses $ 12,600,000 $ 13,500,000 $ 25,400,000 $ 26,900,000  
Mortgages          
Investment [Line Items]          
Number of mortgages or notes on nonaccruals status | loan 0   0   0
Notes          
Investment [Line Items]          
Number of mortgages or notes on nonaccruals status | loan 0   0   0
Secured Debt          
Investment [Line Items]          
Notes receivable $ 4,000,000   $ 4,000,000    
Texas          
Investment [Line Items]          
Number of states exceeding disclosure threshold | State     1    
Percentage of investment in real estate properties 12.00%   12.00%