Equity and Capital - Noncontrolling Interest of Limited Partners (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Limited Partners' Capital Account [Line Items]
Stock Issued During Period, Shares, New Issues	1,730,000	1,899,000
Preferred Stock, Liquidation Preference, Value	$ 325,000,000	$ 325,000,000
Limited Partners' Capital Account, Units Outstanding	154,170	165,796
Partners' Capital Account, Units	94,262,231	92,499,000
General Partners' Capital Account, Units Outstanding	94,108,061	92,333,000
Noncontrolling Interest in Operating Partnerships	31,800,000	19,200,000
Parent Company [Member]
Limited Partners' Capital Account [Line Items]
Stock Issued During Period, Shares, New Issues		13,000,000
Parent Company, Ownership Percentage of Outstanding Common Partnership Units of Operating Partnership	99.80%	99.80%
Series 7 | Parent Company [Member]
Limited Partners' Capital Account [Line Items]
Stock Issued During Period, Shares, New Issues		3,000,000
Preferred Stock, Liquidation Preference, Value	75,000,000	75,000,000
Preferred Stock, Dividend Rate, Percentage	6.00%	6.00%
Preferred Stock, Callable Date	Aug. 23, 2017	Aug. 23, 2017
Series 6 | Parent Company [Member]
Limited Partners' Capital Account [Line Items]
Stock Issued During Period, Shares, New Issues		10,000,000
Preferred Stock, Liquidation Preference, Value	$ 250,000,000	$ 250,000,000
Preferred Stock, Dividend Rate, Percentage	6.625%	6.625%
Preferred Stock, Callable Date	Feb. 16, 2017	Feb. 16, 2017