Equity and Capital - Terms and Conditions of Preferred and Common Stock (Details)		1 Months Ended	12 Months Ended			24 Months Ended
	Feb. 16, 2017	Feb. 26, 2018 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Feb. 06, 2020	Feb. 07, 2018 USD ($)	Feb. 17, 2017 USD ($) $ / shares
Class of Stock [Line Items]
Amount avaiable for issuance			$ 500,000,000
Current Fiscal Year End Date			--12-31
Document Fiscal Year Focus			2017
Series 6 and Series 7
Class of Stock [Line Items]
Award vesting period			5 years
Parent Company [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued | shares			0	13,000,000
Liquidation preference				$ 325,000,000
Net proceeds from common stock issuance				$ 548,920,000	$ 198,494,000
Conversion of Stock, Conversion Ratio			0.45
Stock Issued During Period, Shares, Acquisitions | shares			65,500,000
Parent Company [Member] | Series 6
Class of Stock [Line Items]
Preferred Stock, Date of Issuance			Feb. 16, 2012
Preferred Stock, Shares Issued | shares				10,000,000
Liquidation preference				$ 250,000,000
Preferred Stock, Callable Date				Feb. 16, 2017
Preferred Stock, Redemption Date	Feb. 16, 2017
Preferred Stock, Dividend Rate, Percentage				6.625%
Preferred Stock, Redemption Price Per Share | $ / shares								$ 25.21
Preferred Stock, Redemption Value								$ 252,000,000
Parent Company [Member] | Series 7
Class of Stock [Line Items]
Preferred Stock, Date of Issuance			Aug. 23, 2012
Preferred Stock, Shares Issued | shares				3,000,000
Liquidation preference				$ 75,000,000
Preferred Stock, Callable Date				Aug. 23, 2017
Preferred Stock, Redemption Date	Aug. 23, 2017
Preferred Stock, Dividend Rate, Percentage				6.00%
Preferred Stock, Redemption Price Per Share | $ / shares								$ 25.22
Preferred Stock, Redemption Value								$ 75,700,000.0
Maximum [Member]
Class of Stock [Line Items]
Amount avaiable for issuance			$ 500,000,000
Equity Offering, Common Shares available for issue | shares			3,100,000
Common Stock
Class of Stock [Line Items]
Sale of Stock, Price Per Share | $ / shares			$ 75.25
Subsequent Event [Member]
Class of Stock [Line Items]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased							$ 250,000,000
Stock Repurchase Program Expiration Date						Feb. 06, 2020
Stock Repurchased During Period, Value		$ 74,200,000
Forward Equity Offering [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues | shares			1,850,000
Net proceeds from common stock issuance			$ 137,500,000
Forward Equity Offering 2 [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues | shares			1,250,000
Net proceeds from common stock issuance			$ 89,100,000
ATM Equity Offering Program [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues | shares			0	182,787,000
Weighted Average Price Per Share | $ / shares			$ 0.00	$ 68.85
Net proceeds from common stock issuance			$ 0	$ 12,584,000
Payments for Commissions			0	157,000
Payments of stock issuance costs			$ 349,000	$ 97,000