Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Realized gains:
Total realized gains	$ 2.7	$ 3.0	$ 3.1	$ 6.8
Realized losses:
Total realized losses	0.0	(0.4)	(2.2)	(0.6)
Net realized gain on investments	2.7	2.6	0.9	6.2
Unrealized Gain (Loss) on Investments	7.6	9.5	55.6	(5.8)
Other net realized loss			(1.3)	0.0
Realized Investment Gains (Losses)	(10.3)	(12.1)	(55.2)	(0.4)
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	(7.0)		(14.2)	10.2
Change in net unrealized holding (losses) gains, net of tax	26.2	(9.1)	53.4	(38.4)
Fixed Maturities [Member]
Realized gains:
Total realized gains	1.5	1.3	1.6	1.7
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	33.2	(11.5)	67.6	(48.6)
Equity Securities [Member]
Realized gains:
Total realized gains	1.2	1.7	1.5	5.0
Realized losses:
Unrealized Gain (Loss) on Investments	6.4	10.9	50.5	(3.7)
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	0.0	(0.4)	(2.2)	(0.6)
Other Invested Assets [Member]
Realized gains:
Total realized gains	0.0	0.0	0.0	0.1
Realized losses:
Unrealized Gain (Loss) on Investments	$ 1.2	(1.4)	$ 5.1	$ (2.1)
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax		$ 2.4