Stockholders' Equity (Preferred Stock) (Details) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended	12 Months Ended
	Sep. 12, 2017	Aug. 16, 2017	May 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2012	May 05, 2014	Apr. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred Stock Authorized, but not Issued				10,000,000
Payments for Repurchase of Redeemable Preferred Stock				$ 173	$ 0	$ 0
Preferred Stock, Shares Authorized				10,000,000
Series A Preferred Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred Stock, Capital Shares Reserved for Future Issuance				6,900,000
Preferred Stock, Dividend Rate, Percentage							8.00%
Preferred Stock, Liquidation Preference Per Share									$ 25.00
Payments for Repurchase of Redeemable Preferred Stock		$ 173
Series B Preferred Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred Stock, Percent Interest Per Share			0.10%
Preferred Stock, Capital Shares Reserved for Future Issuance				8,050
Preferred Stock, Dividend Rate, Percentage					7.75%
Preferred Stock, Liquidation Preference Per Share	$ 25.00							$ 25.00
Preferred Stock, Shares Issued				7,000
Series C Preferred Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred Stock, Capital Shares Reserved for Future Issuance				13,800
Preferred Stock, Dividend Rate, Percentage				5.111%	7.00%
Preferred Stock, Shares Issued				13,000
Proceeds from Issuance of Redeemable Preferred Stock	$ 315
Depositary Share [Member] | Series B Preferred Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred Stock, Shares Issued								7,000,000
Depositary Share [Member] | Series C Preferred Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred Stock, Shares Issued	13,000,000