Equity and Capital - Terms and Conditions of Preferred and Common Stock (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jan. 15, 2015
Class of Stock [Line Items]
Document Fiscal Year Focus		2015
Current Fiscal Year End Date		--12-31
Weighted Average Limited Partnership Units Outstanding, Basic		154,170	157,950	171,886
General Partners' Capital Account, Units Outstanding		97,212,638	94,108,000
Partners' Capital Account, Units		97,366,808	94,262,000
Amount avaiable for issuance		$ 83,300,000
Net proceeds from common stock issuance		12,843,000	$ 103,821,000
Payments for Commissions		161,000	1,369,000
Liquidation preference		$ 325,000,000	$ 325,000,000
Equity Offering, Common Shares available for issue					2,875,000
Sale of Stock, Price Per Share					$ 67.40
Equity Offering, Net Proceeds	$ 186,000,000
Weighted Average Price Per Share		$ 67.86	$ 60.00
Limited Partners' Capital Account, Units Outstanding		154,170	154,170
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 common stock issuance		$ 198,494,000	$ 102,453,000	$ 99,753,000
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
Maximum [Member] | Parent Company [Member]
Class of Stock [Line Items]
Award vesting period		5 years