Fair Value Measures (Fair Value of Financial Instruments by Balance Sheet Location) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	$ 7,860,214	$ 8,122,477
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	8,007,406	8,364,245
Level 3 [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes receivable	23,559	24,238
Level 3 [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes receivable	32,008	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	3,049,340	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	3,131,489	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	2,547,452	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	2,578,973	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	983,422	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	1,014,744	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	1,280,000	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	$ 1,282,200	$ 1,536,264