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
	Mar. 31, 2013	Mar. 31, 2012
Schedule of Investments [Line Items]
Credit losses at beginning of period	$ 8.6	$ 14.5
Credit losses for which an OTTI was not previously recognized	0.2	0.1
Additional credit losses on securities for which an OTTI was previously recognized	0.2	0.3
Reductions for securities sold, matured or called	(0.4)	(1.4)
Credit losses at end of period	$ 8.6	$ 13.5