Rollforward of Cumulative Amounts Related to Credit Loss Portion of Other than Temporary Impairments Losses (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Credit losses as of the beginning of the year	$ 8.6	$ 14.5	$ 16.7
Credit losses for which an OTTI was not previously recognized	1.1	0.6
Additional credit losses on securities for which an OTTI was previously recognized	0.3	0.6	0.9
Reductions for securities sold, matured or called	(2.2)	(6.9)	(1.8)
Reductions for securities reclassified as intend to sell		(0.2)	(1.3)
Credit losses as of the end of the year	$ 7.8	$ 8.6	$ 14.5