Equity and Capital - Terms and Conditions of Preferred and Common Stock (Details) (USD $)	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2015	Aug. 10, 2013	Jan. 15, 2015
Class of Stock [Line Items]
Document Fiscal Year Focus	2014
Current Fiscal Year End Date	--12-31
Weighted Average Limited Partnership Units Outstanding, Basic	157,950	171,886	177,164
General Partners' Capital Account, Units Outstanding	94,108,061	92,333,000
Partners' Capital Account, Units	94,262,231	92,499,000
Amount avaiable for issuance	$ 96,000,000	$ 198,400,000
Net proceeds from common stock issuance	103,821,000	101,342,000
Payments for Commissions	1,369,000	1,521,000
Payments of stock issuance costs	0	68,000
Liquidation preference	325,000,000	325,000,000
Weighted Average Price Per Share	$ 60.00	$ 53.35
Limited Partners' Capital Account, Units Outstanding	154,170	165,796
Series 6 and Series 7
Class of Stock [Line Items]
Award vesting period	5 years
Parent Company [Member]
Class of Stock [Line Items]
Parent Company, Ownership Percentage of Outstanding Common Partnership Units of Operating Partnership	99.80%	99.80%
Preferred Stock, Shares Issued	13,000,000
Net proceeds from issuance of preferred stock	0	0	313,900,000
Parent Company [Member] | Series 6
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage	6.625%	6.625%
Preferred Stock, Callable Date	Feb. 16, 2017	Feb. 16, 2017
Preferred Stock, Shares Issued	10,000,000
Liquidation preference	250,000,000	250,000,000
Parent Company [Member] | Series 7
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage	6.00%	6.00%
Preferred Stock, Callable Date	Aug. 23, 2017	Aug. 23, 2017
Preferred Stock, Shares Issued	3,000,000
Liquidation preference	75,000,000	75,000,000
Maximum [Member]
Class of Stock [Line Items]
Amount avaiable for issuance	200,000,000				200,000,000
Maximum [Member] | Parent Company [Member]
Class of Stock [Line Items]
Award vesting period	5 years
Equity Offering [Member]
Class of Stock [Line Items]
Equity Offering, Common Shares available for issue						2,875,000
Sale of Stock, Price Per Share						$ 67.40
Equity Offering, Net Proceeds				$ 193,800,000