Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Realized gains:
Total realized gains	$ 10.0	$ 5.6
Realized losses:
Total realized losses	(2.2)	(4.3)
Net realized gain on investments	7.8	1.3
Change in unrealized holding gains (losses), net of tax:
Deferred federal income tax liability	(8.0)	(12.5)
Change in net unrealized holding (losses) gains, net of tax	15.0	23.2
Fixed Maturities [Member]
Realized gains:
Total realized gains	0.3	1.5
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	5.4	36.2
Fixed Maturities [Member] | OTTI [Member]
Realized losses:
Total realized losses	0.0	(2.3)
Equity Securities [Member]
Realized gains:
Total realized gains	9.7	4.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	14.6	(1.0)
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	(0.1)	(0.6)
Equity Securities [Member] | OTTI [Member]
Realized losses:
Total realized losses	(2.1)	(1.4)
Other Invested Assets [Member]
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	$ 3.0	$ 0.5