Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt Securities, Realized Gain (Loss):
Debt Securities, Available-for-sale, Realized Gain	$ 1.2	$ 0.0	$ 2.8	$ 1.7
Debt Securities, Available-for-sale, Net Realized Gain (Loss)	1.2	0.0	2.8	1.7
Equity Securities, FV-NI, Gain (Loss)
Other net realized loss	0.0	0.0	1.3	0.0
Gain (Loss) on Investments	(5.0)	17.8	50.2	18.2
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	21.4	(14.7)	89.0	(63.3)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	(4.5)	3.1	(18.7)	13.3
Total net unrealized holding gain (loss) on available-for-sale investments	16.9	(11.6)	70.3	(50.0)
Equity Securities [Member]
Equity Securities, FV-NI, Gain (Loss)
Equity Securities, FV-NI, Realized Gain (Loss)	(1.8)	1.0	(2.5)	5.4
Equity Securities, FV-NI, Unrealized Gain (Loss)	(4.1)	16.4	46.4	12.8
Equity Securities, FV-NI, Net Gain (Loss)	(5.9)	17.4	43.9	18.2
Other Invested Assets [Member]
Equity Securities, FV-NI, Gain (Loss)
Equity Securities, FV-NI, Unrealized Gain (Loss)	(0.3)	0.4	4.8	(1.7)
Equity Securities, FV-NI, Net Gain (Loss)	$ (0.3)	$ 0.4	$ 4.8	$ (1.7)