Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Realized gains:
Total realized gains	$ 21.3	$ 9.9	$ 48.0	$ 22.8
Realized losses:
Total realized losses	(1.1)	(1.1)	(4.2)	(6.2)
Net realized gain on investments	20.2	8.8	43.8	16.6
Change in unrealized holding gains (losses), net of tax:
Deferred federal income tax liability	3.2	0.8	(7.6)	(22.8)
Change in net unrealized holding (losses) gains, net of tax	(6.0)	(1.7)	14.2	42.1
Fixed Maturities [Member]
Realized gains:
Total realized gains	0.4	0.0	2.7	2.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	0.4	(11.2)	15.8	54.5
Fixed Maturities [Member] | OTTI [Member]
Realized losses:
Total realized losses	0.0	0.0	0.0	(2.3)
Equity Securities [Member]
Realized gains:
Total realized gains	20.9	9.9	45.2	20.6
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	(12.1)	4.2	(1.9)	6.0
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	(0.5)	0.0	(0.7)	(0.8)
Equity Securities [Member] | OTTI [Member]
Realized losses:
Total realized losses	(0.6)	(1.1)	(3.5)	(3.1)
Other Invested Assets [Member]
Realized gains:
Total realized gains	0.0	0.0	0.1	0.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	$ 2.5	$ 4.5	$ 7.9	$ 4.4