Equity and Capital - Terms and Conditions of Preferred and Common Stock (Details) - USD ($)			12 Months Ended
	Feb. 16, 2017	Jul. 15, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Feb. 17, 2017	Jul. 13, 2016
Class of Stock [Line Items]
Document Fiscal Year Focus			2016
Current Fiscal Year End Date			--12-31
Weighted Average Limited Partnership Units Outstanding, Basic			154,170	154,170	157,950
General Partners' Capital Account, Units Outstanding			104,497,286	97,213,000
Partners' Capital Account, Units			104,651,456	97,367,000
Amount avaiable for issuance			$ 70,800,000
Liquidation preference				$ 325,000,000
Limited Partners' Capital Account, Units Outstanding			154,000	154,000
Series 6 and Series 7
Class of Stock [Line Items]
Award vesting period			5 years
Parent Company [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues				13,000,000
Parent Company, Ownership Percentage of Outstanding Common Partnership Units of Operating Partnership			99.90%	99.80%
Preferred Stock, Shares Issued			13,000,000	13,000,000
Preferred stock, liquidation preferences per share			$ 25	$ 25
Net proceeds from common stock issuance			$ 548,920,000	$ 198,494,000	$ 102,453,000
Liquidation preference			$ 325,000,000
Parent Company [Member] | Series 6
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues				10,000,000
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]
Stock Issued During Period, Shares, New Issues				3,000,000
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
Equity Offering, Common Shares available for issue			3,100,000
Maximum [Member] | Parent Company [Member]
Class of Stock [Line Items]
Award vesting period			5 years
Common Stock
Class of Stock [Line Items]
Sale of Stock, Price Per Share			$ 75.25
Subsequent Event [Member] | Parent Company [Member] | Series 6
Class of Stock [Line Items]
Preferred Stock, Redemption Date	Feb. 16, 2017
Preferred Stock, Redemption Price Per Share						$ 25.21
Preferred Stock, Redemption Value						$ 252,000,000
Equity Offering [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues			5,000,000
Shares Issued, Price Per Share		$ 79.78
Net proceeds from common stock issuance		$ 400,100,000
ATM Equity Offering Program [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues			182,787,000	189,266,000
Net proceeds from common stock issuance			$ 12,584,000	$ 12,843,000
Payments for Commissions			157,000	161,000
Payments of stock issuance costs			$ 97,000	$ 0
Weighted Average Price Per Share			$ 68.85	$ 67.86
Unsecured Debt [Member]
Class of Stock [Line Items]
Unsecured Debt Repayment							$ 300,000,000