Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Realized gains:
Total realized gains	$ 16.7	$ 7.3	$ 26.7	$ 12.9
Realized losses:
Total realized losses	(0.9)	(0.8)	(3.1)	(5.1)
Net realized gain on investments	15.8	6.5	23.6	7.8
Change in unrealized holding gains (losses), net of tax:
Deferred federal income tax liability	(2.8)	(11.1)	(10.8)	(23.6)
Change in net unrealized holding (losses) gains, net of tax	5.2	20.6	20.2	43.8
Fixed Maturities [Member]
Realized gains:
Total realized gains	2.0	0.6	2.3	2.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	10.0	29.5	15.4	65.7
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	14.6	6.6	24.3	10.7
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	(4.4)	2.8	10.2	1.8
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	(0.1)	(0.2)	(0.2)	(0.8)
Equity Securities [Member] | OTTI [Member]
Realized losses:
Total realized losses	0.8	(0.6)	(2.9)	(2.0)
Other Invested Assets [Member]
Realized gains:
Total realized gains	0.1	0.1	0.1	0.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	$ 2.4	$ (0.6)	$ 5.4	$ (0.1)