Commitments and Contingencies (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2010
Financial Guarantee Exposure Eliminated		$ 41.9
Payment to Eliminate Financial Guarantee Exposure		22.1
Weighted Average Contractual Period Remaining For Financial Guarantee Obligations	24.0	13.2
Remaining Individual Financial Guarantee Exposures		49.0
Financial Guarantee Insured Contractual Payments Outstanding		204.8
Financial Guarantee Collateral Cushion Percentage of Exposure		60.00%
Principal [Member]
Financial Guarantee Insured Contractual Payments Outstanding Principal Portion	130.2	198.7
Defaulted, Non-Performing [Member]
Financial Guarantee Insured Contractual Payments Outstanding Default		9.8
Zero Coupon Bonds and Perpetual Securities Backed [Member]
Remaining Individual Financial Guarantee Exposures	108.3	108.3
Collaterized Fund Obligation [Member]
Remaining Individual Financial Guarantee Exposures	14.7	47.5
Government Subsidized Housing Projects [Member]
Remaining Individual Financial Guarantee Exposures	7.2
Legacy Financial Guarantee Business [Member]
Gross Claim Liability Financial Guarantee	1.4
Unearned Premiums Financial Guarantee	0.5
Interest [Member]
Financial Guarantee Insured Contractual Payments Outstanding Interest Portion		6.1
Outstanding Financial Guarantee Contracts [Member]
Gross Claim Liability Financial Guarantee		23.5
Unearned Premiums Financial Guarantee		$ 0.6