Equity and Capital - Terms and Conditions of Preferred and Common Stock (Details) (USD $)	12 Months Ended													12 Months Ended
	Dec. 31, 2013	Aug. 06, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Series 6 and Series 7	Dec. 31, 2013 Parent Company [Member]	Dec. 31, 2012 Parent Company [Member]	Dec. 31, 2011 Parent Company [Member]	Dec. 31, 2013 Parent Company [Member] Series 6	Dec. 31, 2012 Parent Company [Member] Series 6	Dec. 31, 2013 Parent Company [Member] Series 7	Dec. 31, 2012 Parent Company [Member] Series 7	Aug. 10, 2013 Maximum [Member]	Dec. 31, 2013 Maximum [Member] Parent Company [Member]
Class of Stock [Line Items]
Current Fiscal Year End Date	--12-31
Weighted Average Limited Partnership Units Outstanding, Basic	171,886,000		177,164,000	177,164,000
General Partners' Capital Account, Units Outstanding	92,333,161.0		90,394,486
Partners' Capital Account, Units	92,498,957.0		90,571,650
Parent Company, Ownership Percentage of Outstanding Common Partnership Units of Operating Partnership						99.80%	99.80%
Preferred Stock, Dividend Rate, Percentage									6.625%	6.625%	6.00%	6.00%
Preferred Stock, Callable Date									Feb. 16, 2017	Feb. 16, 2017	Aug. 23, 2017	Aug. 23, 2017
Preferred Stock, Shares Issued						13,000,000	13,000,000		10,000,000		3,000,000
Preferred stock, liquidation preferences per share						$ 25	$ 25
Net proceeds from issuance of preferred stock						$ 0	$ 313,900,000	$ 0
Amount avaiable for issuance	198,400,000												200,000,000
Net proceeds from common stock issuance	101,342,000	121,800,000	22,007,000
Payments for Commissions	1,521,000		331,000
Payments of stock issuance costs	68,000		134,000
Liquidation preference	$ 325,000,000		$ 325,000,000						$ 250,000,000	$ 250,000,000	$ 75,000,000	$ 75,000,000
Award vesting period					5 years									5 years
Weighted Average Price Per Share	$ 53.35		$ 49.70
Limited Partners' Capital Account, Units Outstanding	165,796		177,164