Stockholders' Equity (Preferred Stock) (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	12 Months Ended
	Aug. 16, 2017	May 31, 2014	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2012	Sep. 12, 2017	May 05, 2014	Apr. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Document Period End Date			Dec. 31, 2018
Preferred Stock Authorized, but not Issued			10,000,000
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, 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
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, Liquidation Preference Per Share						$ 25.00
Preferred Stock, Shares Issued			13,000
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