Debt Obligations (Shedule of Debt) (Details)
3 Months Ended
Mar. 31, 2018
USD ($)
derivative_instrument
Dec. 31, 2017
USD ($)
derivative_instrument
Debt obligations under various arrangements with financial institutions    
Long-term debt $ 5,651,312,000  
Net unamortized premiums and discounts 884,000  
Net unamortized debt issuance costs (30,085,000)  
Total debt obligations 5,622,111,000 $ 5,676,238,000
Collateral carrying value $ 1,700,000,000  
Interest Rate Swap | Designated as Hedging Instrument    
Debt obligations under various arrangements with financial institutions    
Number of Instruments | derivative_instrument 9 9
Notional Amount $ 1,400,000,000 $ 1,400,000,000
Term Loan | Interest Rate Swap | Designated as Hedging Instrument    
Debt obligations under various arrangements with financial institutions    
Number of Instruments | derivative_instrument 1  
Term Loan | Unsecured $600M Term Loan | 400.0 Million Interest Rate Swaps    
Debt obligations under various arrangements with financial institutions    
Stated spread rate 1.40%  
Effective percentage 0.88%  
Term Loan | Unsecured $600M Term Loan | 400.0 Million Interest Rate Swaps | Designated as Hedging Instrument    
Debt obligations under various arrangements with financial institutions    
Number of Instruments | derivative_instrument 3  
Term Loan | Unsecured $600M Term Loan | 200.0 Million Interest Rate Swaps    
Debt obligations under various arrangements with financial institutions    
Stated spread rate 1.40%  
Effective percentage 0.82%  
Term Loan | Unsecured $600M Term Loan | 200.0 Million Interest Rate Swaps | Designated as Hedging Instrument    
Debt obligations under various arrangements with financial institutions    
Number of Instruments | derivative_instrument 2  
Term Loan | Term Loan, Expires 07/31/2021 | Interest Rate Swap | Designated as Hedging Instrument    
Debt obligations under various arrangements with financial institutions    
Number of Instruments | derivative_instrument 3  
Term Loan | Unsecured $300M Term Loan    
Debt obligations under various arrangements with financial institutions    
Stated spread rate 1.90%  
Notional Amount $ 165,000,000  
Effective percentage 0.82%  
Term Loan | Unsecured $300M Term Loan | Interest Rate Swap | Designated as Hedging Instrument    
Debt obligations under various arrangements with financial institutions    
Number of Instruments | derivative_instrument 1  
Secured Debt    
Debt obligations under various arrangements with financial institutions    
Long-term debt $ 897,859,000 902,717,000
Net unamortized premiums and discounts 13,888,000 15,321,000
Net unamortized debt issuance costs (82,000) (93,000)
Long-term Debt $ 911,665,000 917,945,000
Weighted average fixed interest rate 6.16%  
Secured Debt | Minimum    
Debt obligations under various arrangements with financial institutions    
Stated percentage 4.40%  
Secured Debt | Maximum    
Debt obligations under various arrangements with financial institutions    
Stated percentage 7.89%  
Unsecured Debt    
Debt obligations under various arrangements with financial institutions    
Long-term debt $ 3,218,453,000 3,218,453,000
Net unamortized debt issuance costs (21,678,000) (22,476,000)
Net unamortized discount (13,004,000) (13,485,000)
Long-term Debt $ 3,183,771,000 3,182,492,000
Weighted average fixed interest rate 3.81%  
Unsecured Debt | Minimum    
Debt obligations under various arrangements with financial institutions    
Stated percentage 3.25%  
Unsecured Debt | Maximum    
Debt obligations under various arrangements with financial institutions    
Stated percentage 7.97%  
Unsecured Debt | Unsecured Credit Facility    
Debt obligations under various arrangements with financial institutions    
Long-term debt $ 635,000,000 685,000,000
Stated percentage 3.0375%  
Unsecured Debt | Term Loan    
Debt obligations under various arrangements with financial institutions    
Stated spread rate 1.35%  
Term loan face amount $ 135,000,000.0  
Effective percentage 0.82%  
Unsecured Debt | Term Loan | Unsecured $600M Term Loan    
Debt obligations under various arrangements with financial institutions    
Long-term debt $ 600,000,000 600,000,000
Stated percentage 3.0875%  
Term loan face amount $ 600,000,000  
Unsecured Debt | Term Loan | Unsecured $600M Term Loan | 400.0 Million Interest Rate Swaps    
Debt obligations under various arrangements with financial institutions    
Notional Amount 400,000,000  
Unsecured Debt | Term Loan | Unsecured $600M Term Loan | 200.0 Million Interest Rate Swaps    
Debt obligations under various arrangements with financial institutions    
Notional Amount $ 200,000,000  
Unsecured Debt | Term Loan | Term Loan, Expires 07/31/2021    
Debt obligations under various arrangements with financial institutions    
Stated spread rate 1.35%  
Term loan face amount $ 500,000,000.0  
Effective percentage 1.11%  
Unsecured Debt | Term Loan | Unsecured $300M Term Loan    
Debt obligations under various arrangements with financial institutions    
Long-term debt $ 300,000,000 300,000,000
Stated percentage 3.56418%  
Term loan face amount $ 300,000,000  
Unsecured Debt | Unsecured Credit Facility and Term Loan    
Debt obligations under various arrangements with financial institutions    
Net unamortized debt issuance costs (8,325,000) (9,199,000)
Long-term Debt $ 1,526,675,000 $ 1,575,801,000