Fair Value Measures - Fair value, by balance sheet grouping (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instruments	$ 5,957,640	$ 5,745,446
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instruments	6,508,078	5,990,239
Level 2 [Member] | Mortgage Notes Payable and other debt, net [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instruments	1,334,689	1,535,918
Level 2 [Member] | Mortgage Notes Payable and other debt, net [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instruments	1,384,490	1,590,915
Level 2 [Member] | Corporate Bonds, Net [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instruments	4,622,951	2,839,581
Level 2 [Member] | Corporate Bonds, Net [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instruments	5,123,588	3,022,087
Level 2 [Member] | Convertible Debt, Net [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instruments	0	319,947
Level 2 [Member] | Convertible Debt, Net [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instruments	0	327,237
Level 2 [Member] | Credit Facility [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instruments	0	1,050,000
Level 2 [Member] | Credit Facility [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instruments	$ 0	$ 1,050,000