Equity and Capital - Noncontrolling Interest of Limited Partners (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Limited Partners' Capital Account [Line Items]
Stock Issued During Period, Shares, New Issues	189,266,000	1,730,363,000
Preferred Stock, Liquidation Preference, Value	$ 325,000,000	$ 325,000,000
Limited Partners' Capital Account, Units Outstanding	154,170	154,170
Partners' Capital Account, Units	97,366,808	94,262,000
General Partners' Capital Account, Units Outstanding	97,212,638	94,108,000
Noncontrolling Interest in Operating Partnerships	$ 30,500,000	$ 31,800,000
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