Shareholders' Equity (Narrative) (Details) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 shares
Schedule of Equity Method Investments [Line Items]
Current Fiscal Year End Date			--12-31
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax			$ 19.0	$ (45.0)
Liquidation preference of preferred stock (per share) | $ / shares			$ 1,000
Preferred stock, authorized shares | shares			4,400	4,400
Carrying value, Preferred stock			$ 566.0	$ 710.0
Common Stock			$ 4,445.0	$ 4,725.0
Common Stock, Shares, Issued | shares			197,532	203,085
Warrants sold | shares			29,300
Common stock issued for preferred stock redemption			$ 144.0	$ (126.0)	$ (176.0)
Preferred Stock Redemption Premium			$ 2.0
Stock Repurchased During Period, Shares | shares			7,000	3,000
Stock Repurchased and Retired During Period, Value	$ (115.0)		$ 320.0	$ 90.0
Treasury Stock Acquired, Average Cost Per Share | $ / shares	$ 53.46		$ 45.66	$ 31.15
Treasury Stock Acquired, Repurchase Authorization	120
Preferred Stock, Tender Offer				$ 120.0
Preferred Stock, Liquidation Preference, Value				$ 118.0
Payments for Repurchase of Redeemable Preferred Stock			$ 126.0
Preferred Stock Redemption			$ 10.0
Common stock, outstanding shares | shares			197,532	203,085
Total invested assets of trusts			$ 102.0	$ 91.0
Total obligations of trusts			116.0	105.0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax			(1.0)	0.0
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax			$ 11.0	5.0
Preferred stock, liquidation preference per depositary share | $ / shares			$ 25
Common Stock Held in Trust			$ 14.0	14.1
Warrant Exercise Price | $ / shares			$ 35.37
Depositary Share, Preferred Stock Ownership Interest			1/40
Troubled Asset Relief Program, Stock		$ 1.0
Effect of Change in Tax Rate, OCI			$ 25.0	0.0	0.0
Series I Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred stock, authorized shares | shares			300,893
Carrying value, Preferred stock			$ 99.0	99.0
Preferred Stock, Dividend Rate, Percentage			5.80%
Series A Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred stock, authorized shares | shares			140,000
Variable dividend payment rate			> of 4.0% or 3mL+0.52%
Carrying value, Preferred stock			$ 67.0	67.0
Series F Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred stock, authorized shares | shares			0
Carrying value, Preferred stock			$ 0.0	144.0
Preferred Stock, Dividend Rate, Percentage			7.90%
Series G Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred stock, authorized shares | shares			200,000
Carrying value, Preferred stock			$ 138.0	138.0
Preferred Stock, Dividend Rate, Percentage			6.30%
Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Common stock issued for preferred stock redemption			$ 144.0	(118.0)	(176.0)
Retained Earnings [Member]
Schedule of Equity Method Investments [Line Items]
Common stock issued for preferred stock redemption			$ 2.0	(10.0)	(3.0)
Warrant [Member]
Schedule of Equity Method Investments [Line Items]
Common stock warrants issued to the U.S. Treasury			5,800,000
Common Stock [Member]
Schedule of Equity Method Investments [Line Items]
Common stock issued for preferred stock redemption			$ (2.0)	$ 2.0	$ 3.0
Stock Repurchased During Period, Shares | shares			(7,009)	(2,889)
Stock Repurchased and Retired During Period, Value			$ 320.0
Common stock, outstanding shares | shares			197,532	203,085	204,417	203,015
Warrant Exercise Price [Member]
Schedule of Equity Method Investments [Line Items]
Common stock warrants issued to the U.S. Treasury			0
Series G Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred Stock, Liquidation Preference, Value				$ 33.0
Series I Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred Stock, Liquidation Preference, Value				26.0
SERIES J STOCK [Member]
Schedule of Equity Method Investments [Line Items]
Preferred Stock, Liquidation Preference, Value				$ 59.0
Future [Member]
Schedule of Equity Method Investments [Line Items]
Troubled Asset Relief Program, Stock		29.0
Warrant [Member]
Schedule of Equity Method Investments [Line Items]
Troubled Asset Relief Program, Stock		3.2
Next Twelve Months [Member]
Schedule of Equity Method Investments [Line Items]
Troubled Asset Relief Program, Stock		$ 2.6