Derivative Instruments (Details) - Narrative (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2012 contracts	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Derivative [Line Items]
Derivative, Collateral, Obligation to Return Cash		$ 124.7		$ 124.7		$ 77.1
Increase (Decrease) in Accrued Investment Income Receivable		2.5	2.6	4.9	5.1
Derivatives Number of Instruments Settled	2
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