Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Realized gains:
Total realized gains	$ 5.6	$ 5.2
Realized losses:
Total realized losses	(4.3)	(1.0)
Net realized gain on investments	1.3	4.2
Change in unrealized holding gains (losses), net of tax:
Deferred federal income tax liability	(12.5)	(4.5)
Change in net unrealized holding (losses) gains, net of tax	23.2	8.3
Fixed Maturities [Member]
Realized gains:
Total realized gains	1.5	1.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	36.2	9.4
Fixed Maturities [Member] | OTTI [Member]
Realized losses:
Total realized losses	(2.3)	0.0
Equity Securities [Member]
Realized gains:
Total realized gains	4.1	4.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	(1.0)	(1.0)
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	(0.6)	(0.5)
Equity Securities [Member] | OTTI [Member]
Realized losses:
Total realized losses	(1.4)	(0.5)
Other Invested Assets [Member]
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	$ 0.5	$ 4.4