Fair Value Measures (Fair Value of Financial Instruments by Balance Sheet Location) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	$ 8,544,019	$ 8,122,477
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	8,727,223	8,364,245
Level 3 [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes receivable	23,297	24,238
Level 3 [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes receivable	32,848	31,842
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	2,956,737	3,133,005
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	3,072,968	3,240,153
Level 2 [Member] | Corporate Bonds, Net [Member] | Senior Unsecured Note [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	3,547,648	2,547,255
Level 2 [Member] | Corporate Bonds, Net [Member] | Senior Unsecured Note [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	3,585,672	2,580,786
Level 2 [Member] | Convertible Debt, Net [Member] | Convertible Senior Notes [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	984,634	982,217
Level 2 [Member] | Convertible Debt, Net [Member] | Convertible Senior Notes [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	1,012,998	1,007,042
Level 2 [Member] | Credit Facility [Member] | Credit Facilities [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	1,055,000	1,460,000
Level 2 [Member] | Credit Facility [Member] | Credit Facilities [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	$ 1,055,585	$ 1,536,264