Investments - Realized Losses Related to Other-than-Temporary Impairments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (0.5)	$ (0.8)
Large-Cap Equity Securities [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	0	(0.2)
Small-Cap Securities [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (0.5)	$ (0.6)