Capital Preferred Stock (Additional Information) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2009	Nov. 30, 2017	May 31, 2017	Nov. 30, 2014	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Sep. 22, 2011	Dec. 31, 2008	Dec. 31, 2006	Sep. 30, 2006
Class of Stock [Line Items]
Preferred Stock, Shares Authorized	50,000,000	50,000,000
Stock Issued During Period, Value, New Issues	$ 1,239,000,000
Payments for Repurchase of Warrants	$ 0	$ 24,000,000	$ 0
Class of Warrant or Right, Outstanding	7,100,000
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Authorized														5,000
Preferred Stock, Shares Issued														5,000
Preferred Stock, No Par Value														$ 0
Preferred Stock, Liquidation Preference, Value														$ 100,000
Declared And Accrued Preferred Stock Dividend Basis Spread on Variable Rate	0.53%
Declared And Accrued Preferred Stock Dividend Fixed Rate	4.00%
Stock Redeemed or Called During Period, Shares				3,275
Preferred Stock, Redemption Price Per Share										$ 100,000
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Authorized													5,010
Preferred Stock, Shares Issued									1,025
Preferred Stock, No Par Value									$ 0
Preferred Stock, Liquidation Preference, Value									$ 100,000
Declared And Accrued Preferred Stock Dividend Basis Spread on Variable Rate	0.65%
Declared And Accrued Preferred Stock Dividend Fixed Rate	4.00%
Preferred Stock, Redemption Price Per Share									$ 100,000
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued												35,000
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued												13,500
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Authorized								5,000
Preferred Stock, Shares Issued								4,500
Preferred Stock, No Par Value								$ 0
Preferred Stock, Liquidation Preference, Value								$ 100,000
Declared And Accrued Preferred Stock Dividend Fixed Rate	5.88%
Preferred Stock, Redemption Price Per Share								$ 100,000
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Authorized							5,000
Preferred Stock, Shares Issued							5,000
Preferred Stock, No Par Value							$ 0
Preferred Stock, Liquidation Preference, Value							$ 100,000
Declared And Accrued Preferred Stock Dividend Basis Spread on Variable Rate	3.86%
Declared And Accrued Preferred Stock Dividend Fixed Rate	5.63%
Preferred Stock, Redemption Price Per Share							$ 100,000
Series G Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Authorized						7,500
Preferred Stock, Shares Issued						7,500
Preferred Stock, No Par Value						$ 0
Preferred Stock, Liquidation Preference, Value						$ 100,000
Declared And Accrued Preferred Stock Dividend Basis Spread on Variable Rate	3.10%
Declared And Accrued Preferred Stock Dividend Fixed Rate	5.05%
Series H Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Authorized					5,000
Preferred Stock, Shares Issued					5,000
Preferred Stock, No Par Value					$ 0
Preferred Stock, Liquidation Preference, Value					$ 100,000
Declared And Accrued Preferred Stock Dividend Basis Spread on Variable Rate	2.79%
Declared And Accrued Preferred Stock Dividend Fixed Rate	5.13%
Preferred Stock, Redemption Price Per Share					$ 100,000
Warrant [Member]
Class of Stock [Line Items]
Class of Warrant or Right, Outstanding												17,900,000
Series A [Member]
Class of Stock [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights											4,000,000	6,000,000
Class of Warrant or Right, Exercise Price of Warrants or Rights											$ 11,000,000	$ 33.70
warrants purchased		1,100,000
Series B [Member]
Class of Stock [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights												11,900,000
Class of Warrant or Right, Exercise Price of Warrants or Rights												$ 44.15
warrants purchased		5,400,000
Parent Company [Member]
Class of Stock [Line Items]
Payments for Repurchase of Warrants	$ 0	$ 24,000,000	$ 0