Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Debt Securities, Realized Gain (Loss):
Debt Securities, Available-for-sale, Realized Gain	$ 1.3	$ 1.2	$ 6.8	$ 2.8
Debt Securities, Available-for-sale, Realized Loss	(1.5)	0.0	(3.6)	0.0
Debt Securities, Available-for-sale, Net Realized Gain (Loss)	(0.2)	1.2	3.2	2.8
Equity Securities, FV-NI, Gain (Loss)
Other net realized loss	0.0	0.0	(0.2)	1.3
Gain (Loss) on Investments	20.0	(5.0)	(39.3)	50.2
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	4.8	21.4	71.3	89.0
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	(1.1)	(4.5)	(15.0)	(18.7)
Total net unrealized holding gains on available-for-sale investments	3.7	16.9	56.3	70.3
Equity Securities [Member]
Equity Securities, FV-NI, Gain (Loss)
Equity Securities, FV-NI, Realized Gain (Loss)	(42.3)	(1.8)	(50.9)	(2.5)
Equity Securities, FV-NI, Unrealized Gain (Loss)	60.5	(4.1)	15.8	46.4
Equity Securities, FV-NI, Net Gain (Loss)	18.2	(5.9)	(35.1)	43.9
Other Invested Assets [Member]
Equity Securities, FV-NI, Gain (Loss)
Equity Securities, FV-NI, Unrealized Gain (Loss)	2.0	(0.3)	(7.6)	4.8
Equity Securities, FV-NI, Net Gain (Loss)	$ 2.0	$ (0.3)	$ (7.6)	$ 4.8