Schedule III Part II - Summary of Real Estate and Accumulated Depreciation (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
SEC Schedule III, Real Estate and Accumulated Depreciation Disclosure [Abstract]
Aggregate cost basis for Federal tax purposes	$ 5,100,000,000
Real Estate and Accumulated Depreciation, Carrying Amount of Land and Buildings and Improvements [Abstract]
Balance at beginning of year	5,149,463,000	4,779,674,000	4,426,444,000
Acquisitions	174,328,000	76,359,000	193,131,000
Improvements	329,674,000	329,522,000	187,990,000
Deduction during period—disposition and retirements of property and transfer to joint venture	(44,467,000)	(36,092,000)	(27,891,000)
Balance at end of year	5,608,998,000	5,149,463,000	4,779,674,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at beginning of year	1,350,471,000	1,224,295,000	1,127,588,000
Additions during period—depreciation and amortization expense	155,662,000	147,730,000	128,654,000
Deductions during period—disposition and retirements of property	(39,083,000)	(21,554,000)	(31,947,000)
Balance at end of year	$ 1,467,050,000	$ 1,350,471,000	$ 1,224,295,000