Fair Value Measures - Fair Value of Financial Instruments by Balance Sheet Location (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Carrying Amount [Member]
Liabilities:
Total liabilities	$ 6,422,908	$ 8,122,477
Fair Value [Member]
Liabilities:
Total liabilities	6,491,738	8,364,245
Level 3 [Member] | Carrying Amount [Member]
Assets:
Mortgage notes receivable	22,764	24,238
Level 3 [Member] | Fair Value [Member]
Assets:
Mortgage notes receivable	30,460	31,842
Level 2 [Member] | Mortgages [Member] | Carrying Amount [Member]
Liabilities:
Total liabilities	2,687,739	3,133,005
Level 2 [Member] | Mortgages [Member] | Fair Value [Member]
Liabilities:
Total liabilities	2,713,155	3,240,153
Level 2 [Member] | Corporate Bonds, Net [Member] | Senior Unsecured Note [Member] | Carrying Amount [Member]
Liabilities:
Total liabilities	2,248,063	2,547,255
Level 2 [Member] | Corporate Bonds, Net [Member] | Senior Unsecured Note [Member] | Fair Value [Member]
Liabilities:
Total liabilities	2,273,850	2,580,786
Level 2 [Member] | Convertible Debt, Net [Member] | Convertible Senior Notes [Member] | Carrying Amount [Member]
Liabilities:
Total liabilities	987,106	982,217
Level 2 [Member] | Convertible Debt, Net [Member] | Convertible Senior Notes [Member] | Fair Value [Member]
Liabilities:
Total liabilities	1,004,733	1,007,042
Level 2 [Member] | Credit Facility [Member] | Credit Facilities [Member] | Carrying Amount [Member]
Liabilities:
Total liabilities	500,000	1,460,000
Level 2 [Member] | Credit Facility [Member] | Credit Facilities [Member] | Fair Value [Member]
Liabilities:
Total liabilities	$ 500,000	$ 1,536,264