Investments - Realized Losses Related to Other-than-Temporary Impairments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (0.6)	$ (1.1)	$ (3.5)	$ (5.4)
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	0.0	0.0	0.0	(2.3)
Large-Cap Equity Securities [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	0.0	0.0	(1.5)	(0.2)
Small-Cap Securities [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (0.6)	$ (1.1)	$ (2.0)	$ (2.9)