Investments (Rollforward of Cumulative Amounts Related to Credit Loss Portion of OTTI Losses) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Investments [Abstract]
Credit losses at beginning of period	$ 5.3	$ 8.6	$ 7.8	$ 8.6
Credit losses for which an OTTI was not previously recognized		0.1		0.3
Additional credit losses on securities for which an OTTI was previously recognized				0.2
Reductions for securities sold, matured or called	(0.6)	(1.4)	(3.1)	(1.8)
Reductions for securities reclassified as intended to sell	(0.4)		(0.4)
Credit losses at end of period	$ 4.3	$ 7.3	$ 4.3	$ 7.3