Investments - Realized and Unrealized Holding Gains (Losses) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Realized gains:
Total realized gains	$ 5.2	$ 11.6
Realized losses:
Total realized losses	(1.0)	(0.9)
Net realized gain on investments	4.2	10.7
Change in unrealized holding gains (losses), net of tax:
Deferred federal income tax liability	(4.5)	(4.3)
Valuation allowance	0	0.1
Change in net unrealized holding (losses) gains, net of tax	8.3	8.1
Fixed Maturities [Member]
Realized gains:
Total realized gains	1.1	2.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	9.4	14.4
Equity Securities [Member]
Realized gains:
Total realized gains	4.1	9.4
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	(1.0)	(4.5)
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	(0.5)	(0.1)
Equity Securities [Member] | OTTI [Member]
Realized losses:
Total realized losses	(0.5)	(0.8)
Other Invested Assets [Member]
Realized gains:
Total realized gains	0	0.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	$ 4.4	$ 2.4