Rollforward of Cumulative Amounts Related to Credit Loss Portion of Other than Temporary Impairments Losses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Schedule of Investments [Line Items]
Credit losses as of the beginning of the period	$ 13.5	$ 16.3	$ 14.5	$ 16.7
Credit losses for which an OTTI was not previously recognized	0.3		0.4
Additional credit losses on securities for which an OTTI was previously recognized	0.3		0.6	0.2
Reduction for securities sold, matured or called during the period	(0.3)	(0.7)	(1.7)	(1.3)
Reduction for securities reclassified as intend to sell		(0.8)		(0.8)
Credit losses as of the end of the period	$ 13.8	$ 14.8	$ 13.8	$ 14.8