Derivative Instruments (Details) - Narrative (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Jul. 31, 2012 contracts	Dec. 31, 2012 contracts	Dec. 31, 2011 contracts	Dec. 31, 2010 contracts
Derivative [Line Items]
Derivative, Collateral, Obligation to Return Cash			$ 77.1
Derivative Cash Collateral Paid		12.1
Deficiency Guarantee Government Backing Percent		80.00%
Derivatives Number of Instruments Settled	2			3
Weighted average years to maturity		16 years 1 month 6 days
Periodic coupon settlement increase to Net investment income		0	0	2.3
Periodic coupon settlement decrease to interest expense		4.9	10.2	49.8
Number of instruments held		1	2
Notes Payable And Debt [Member]
Derivative [Line Items]
Cumulative increase to carrying value				21.6
Deposit Liabilities [Member]
Derivative [Line Items]
Cumulative increase to carrying value	83.7			$ 149.5