Fair Value Measures - Fair value, by balance sheet grouping (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	$ 6,006,205	$ 5,745,446
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	6,191,604	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]
Total liabilities	1,400,317	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]
Total liabilities	1,446,194	1,590,915
Level 2 [Member] | Corporate Bonds, Net [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	3,435,016	2,839,581
Level 2 [Member] | Corporate Bonds, Net [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	3,575,898	3,022,087
Level 2 [Member] | Convertible Debt, Net [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	270,872	319,947
Level 2 [Member] | Convertible Debt, Net [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	269,512	327,237
Level 2 [Member] | Credit Facility [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	900,000	1,050,000
Level 2 [Member] | Credit Facility [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	$ 900,000	$ 1,050,000