Derivative Instruments (Details) - Narrative (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 31, 2012 contracts	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2010 contracts	Dec. 31, 2011
Derivative [Line Items]
Derivative, Collateral, Obligation to Return Cash							$ 77.1
Derivative Cash Collateral Paid		41.5		41.5
Periodic coupon settlement reduction to interest expense		0	2.6	4.9	7.7
Derivatives Number of Instruments Settled	2					3
Notes Payable And Debt [Member]
Derivative [Line Items]
Cumulative increase to carrying value		21.6		21.6
Deposit Liabilities [Member]
Derivative [Line Items]
Cumulative increase to carrying value	83.7	$ 149.5		$ 149.5
Weighted average years to maturity	16 years 1 month 6 days