Investments - Realized and Unrealized Holding Gains (Losses) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Realized gains:
Total realized gains	$ 5.6	$ 4.1	$ 17.3	$ 11.2
Realized losses:
Total realized losses	(0.3)	(2.6)	(1.3)	(3.0)
Net realized gain on investments	5.3	1.5	16.0	8.2
Change in unrealized holding gains (losses), net of tax:
Deferred federal income tax liability	(10.3)	24.0	(14.6)	19.6
Valuation allowance	6.8	(16.7)	6.9	(12.0)
Change in net unrealized holding gains, net of tax	26.1	(61.3)	34.2	(48.3)
Fixed Maturities [Member]
Realized gains:
Total realized gains	0.4	0.7	2.5	1.2
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	19.6	(69.3)	34.0	(78.5)
Equity Securities [Member]
Realized gains:
Total realized gains	5.2	3.4	14.7	10.0
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	6.2	(0.3)	1.7	18.0
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	0	(0.6)	(0.1)	(0.7)
Equity Securities [Member] | OTTI [Member]
Realized losses:
Total realized losses	(0.3)	(2.0)	(1.2)	(2.3)
Other Invested Assets [Member]
Realized gains:
Total realized gains	0	0	0.1	0
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	$ 3.8	$ 1.0	$ 6.2	$ 4.6