Stockholders' Equity (Preferred Stock) (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	May 31, 2014	Jun. 30, 2012	Dec. 31, 2014	Dec. 31, 2012	Sep. 30, 2016	Dec. 31, 2015	May 05, 2014	Apr. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred Stock, Shares Authorized					10,000,000	10,000,000
Preferred Stock Authorized, but not Issued					3,100,000
Preferred Stock, Shares Issued					6,900,000	6,900,000
Preferred Stock, Value, Issued					$ 336	$ 336
Preferred Class A [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred Stock, Capital Shares Reserved for Future Issuance					6,900,000
Sale of Stock, Price Per Share								$ 24.2125
Series A Preferred Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred Stock, Dividend Rate, Percentage				8.00%
Issuance of preferred stock		$ 167
Preferred Stock, Liquidation Preference Per Share								$ 25.00
Preferred Class B [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred Stock, Capital Shares Reserved for Future Issuance					8,050
Preferred Stock, Dividend Rate, Percentage			7.75%
Preferred Stock, Shares Issued							7,000,000
Share Price							$ 24.2125
Preferred Stock, Value, Issued							$ 169
Series B Preferred Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred Stock, Liquidation Preference Per Share							$ 25.00
Preferred stock, percent interest per share	0.10%