Trust-Preferred Securities and Debt Extinguishment (Details) (USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Jan. 31, 2011
Schedule Of Trust Preferred Securities [Line Items]
Maximum deferred period of payment of interest	5 years
Accrued interest payable on trust preferred securities	$ 33.3		$ 27.6
Number of previously unissued common shares, issued (in shares)		19,545,360
Trust preferred securities liquidation value, aggregate				19.5
Accrued interest payable associated with retired securities				3.4
Amount by which regulatory capital increased				21.9
Amount of gain on transaction of regulatory capital increased		$ 16.9
Trust Preferred Securities [Member] | Capitol Trust I [Member]
Schedule Of Trust Preferred Securities [Line Items]
Number of exchangeable shares (in shares)				1,180,602
Number of shares outstanding as trust preferred securities (in shares)				2,530,000
Trust Preferred Securities [Member] | Capitol Trust XII [Member]
Schedule Of Trust Preferred Securities [Line Items]
Number of exchangeable shares (in shares)				773,934
Number of shares outstanding as trust preferred securities (in shares)				1,454,100