Equity and Capital - Noncontrolling Interest of Limited Partners (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Limited Partners' Capital Account [Line Items]
Stock Issued During Period, Shares, New Issues	1,899,000	443,000
Preferred Stock, Liquidation Preference, Value	$ 325,000,000	$ 325,000,000
Limited Partners' Capital Account, Units Outstanding	165,796	177,164
Partners' Capital Account, Units	92,498,957.0	90,571,650
General Partners' Capital Account, Units Outstanding	92,333,161.0	90,394,486
Noncontrolling Interest in Operating Partnerships	19,200,000	16,300,000
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