Fair Value Measures - Fair value, by balance sheet grouping (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	$ 6,344,497	$ 5,745,446
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	6,268,083	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	1,412,894	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	1,459,616	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	2,839,865	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	2,717,813	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	320,425	319,947
Level 2 [Member] | Convertible Debt, Net [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	319,341	327,237
Level 2 [Member] | Credit Facility [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	1,771,313	1,050,000
Level 2 [Member] | Credit Facility [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	$ 1,771,313	$ 1,050,000