Fair Value Measures - Fair value, by balance sheet grouping (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	$ 5,700,345	$ 6,130,685
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	5,927,953	6,130,329
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	1,754,571	1,933,209
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	1,814,875	1,961,496
Level 2 [Member] | Corporate Bonds, Net [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	2,646,213	3,395,885
Level 2 [Member] | Corporate Bonds, Net [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	2,806,706	3,368,928
Level 2 [Member] | Convertible Debt, Net [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	399,561	398,591
Level 2 [Member] | Convertible Debt, Net [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	406,372	396,905
Level 2 [Member] | Credit Facility [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	900,000	403,000
Level 2 [Member] | Credit Facility [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	$ 900,000	$ 403,000