Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Realized gains:
Total realized gains	$ 1.0	$ 21.3	$ 7.7	$ 48.0
Realized losses:
Total realized losses	0.0	(1.1)	(0.6)	(4.2)
Net realized gain on investments	1.0	20.2	7.1	43.8
Unrealized Gain (Loss) on Investments	16.8	0.0	11.1	0.0
Realized Investment Gains (Losses)	(17.8)	(20.2)	(18.2)	(43.8)
Change in unrealized holding gains (losses), net of tax:
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	3.1	3.2	13.3	(7.6)
Change in net unrealized holding (losses) gains, net of tax	(11.6)	(6.0)	(50.0)	14.2
Fixed Maturities [Member]
Realized gains:
Total realized gains	0.0	0.4	1.7	2.7
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	(14.7)	0.4	(63.3)	15.8
Equity Securities [Member]
Realized gains:
Total realized gains	1.0	20.9	6.0	45.2
Realized losses:
Unrealized Gain (Loss) on Investments	16.4	0.0	12.8	0.0
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	0.0	(12.1)	0.0	(1.9)
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	0.0	(0.5)	(0.6)	(0.7)
Equity Securities [Member] | Other Than Temporary Impairments [Member]
Realized losses:
Total realized losses	0.0	(0.6)	0.0	(3.5)
Other Invested Assets [Member]
Realized gains:
Total realized gains	0.0	0.0	0.0	0.1
Realized losses:
Unrealized Gain (Loss) on Investments	0.4	0.0	(1.7)	0.0
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	$ 0.0	$ 2.5	$ 0.0	$ 7.9