Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended				6 Months Ended	8 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Dec. 30, 2010	Jun. 30, 2011 Deposit Liability Contracts [Member]	Jun. 30, 2011 Deposit Liability Contracts [Member]	Jun. 30, 2010 Notes Payable And Debt [Member]	Jun. 30, 2011 Structured Finance [Member]	Dec. 31, 2010 Structured Finance [Member]	Jun. 30, 2011 Collateralized Debt Obligations [Member]
Cash Collateral for Borrowed Securities	$ 32.9		$ 32.9		$ 23.0
Gain on Settlement of Derivative Part of Fair Value Hedge								149.5	21.6
Interest Expense Reduction							5.7		7.4
Gain on Settlement of Derivative Part of Fair Value Hedge Remaining to be Amortized							143.8		14.2
Remaining Period for Gain on Settlement of Derivative Part of Fair Value Hedge Remaining to be Amortized			2.9				35.0
Increase (Decrease) in Accrued Investment Income Receivable		1.0		2.0
Increase (Decrease) in Interest Payable, Net	2.6	20.5	5.1	34.5
Notional Amount Of Derivative Instruments Designated As Investment Hedges	1,600.0	505.4	1,500.0	372.7
Gain (Loss) on Derivative Used in Net Investment Hedge, Net of Tax	27.8	19.9	69.2	22.6
Schedule of Insured Financial Obligations with Credit Deterioration, Insured Contractual Payments Outstanding	175.2		175.2								246.3
Schedule of Insured Financial Obligations with Credit Deterioration, Insured Contractual Payments Outstanding, Principal										159.3	226.4
Schedule of Insured Financial Obligations with Credit Deterioration, Insured Contractual Payments Outstanding, Interest										15.9	19.9
Derivative, Remaining Maturity										4.8	5.3
Weighted Average Credit Rating Of Credit Derivatives Outstanding	B-		B-		B-
Deficiency Guarantee Government Backing Percent						80.00%
Derivative, Net Liability Position, Aggregate Fair Value										28.5	25.9
Collateral Paid In Retation To Derivative Agreement												$ 4.0