Investments - Realized and Unrealized Holding Gains (Losses) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Realized gains:
Total realized gains	$ 3.3	$ 8.4	$ 20.6	$ 19.6
Realized losses:
Total realized losses	(1.7)	(0.9)	(3.0)	(3.9)
Net realized gain on investments	1.6	7.5	17.6	15.7
Change in unrealized holding gains (losses), net of tax:
Deferred federal income tax liability	6.2	(1.6)	(8.4)	18.0
Valuation allowance	(6.8)	3.0	0.1	(9.0)
Change in net unrealized holding (losses) gains, net of tax	(18.5)	6.0	15.7	(42.3)
Fixed Maturities [Member]
Realized gains:
Total realized gains	0.1	0.4	2.6	1.6
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	(8.6)	(7.7)	25.4	(86.2)
Fixed Maturities [Member] | Sales [Member]
Realized losses:
Total realized losses	0	0.3	0	0.3
Equity Securities [Member]
Realized gains:
Total realized gains	3.2	7.9	17.9	17.9
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	(4.7)	5.3	(3.0)	23.3
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	(0.6)	(0.3)	(0.7)	(1.0)
Equity Securities [Member] | OTTI [Member]
Realized losses:
Total realized losses	(1.1)	(0.3)	(2.3)	(2.6)
Other Invested Assets [Member]
Realized gains:
Total realized gains	0	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	$ (4.6)	$ 7.0	$ 1.6	$ 11.6