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Debt (Summary of Senior Notes) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Joint venture debt company proportionate share total $ 57,000  
Net premiums, discounts, and issuance costs (13,105) $ (13,057)
Notes payable 2,986,895 2,986,943
Other Notes Payable 100,823 20,024
Notes Payable 3,087,718 3,006,967
Notes Payable, Fair Value Disclosure 3,076,140 3,263,774
4.250% unsecured senior notes due March 2021 | Senior Notes    
Debt Instrument [Line Items]    
Face amount $ 700,000 $ 700,000
Stated interest rate 4.25% 4.25%
5.500% unsecured senior notes due March 2026 | Senior Notes    
Debt Instrument [Line Items]    
Face amount $ 700,000 $ 700,000
Stated interest rate 5.50% 5.50%
5.000% unsecured senior notes due January 2027 | Senior Notes    
Debt Instrument [Line Items]    
Face amount $ 600,000 $ 600,000
Stated interest rate 5.00% 5.00%
7.875% unsecured senior notes due June 2032 | Senior Notes    
Debt Instrument [Line Items]    
Face amount $ 300,000 $ 300,000
Stated interest rate 7.875% 7.875%
6.375% unsecured senior notes due May 2033 | Senior Notes    
Debt Instrument [Line Items]    
Face amount $ 400,000 $ 400,000
Stated interest rate 6.375% 6.375%
6.000% unsecured senior notes due February 2035 | Senior Notes    
Debt Instrument [Line Items]    
Face amount $ 300,000 $ 300,000
Stated interest rate 6.00% 6.00%