Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Debt Securities, Realized Gain (Loss):
Debt Securities, Available-for-sale, Realized Gain	$ 3.6	$ 0.1
Debt Securities, Available-for-sale, Realized Loss	(1.2)	0.0
Debt Securities, Available-for-sale, Net Realized Gain (Loss)	2.4	0.1
Equity Securities, FV-NI, Gain (Loss)
Other net realized loss	(0.2)	1.3
Gain (Loss) on Investments	(135.2)	44.9
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	28.6	34.4
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	(6.0)	(7.2)
Total net unrealized holding gain on available-for-sale investments	22.6	27.2
Equity Securities [Member]
Equity Securities, FV-NI, Gain (Loss)
Equity Securities, FV-NI, Realized Gain (Loss)	0.9	(1.9)
Equity Securities, FV-NI, Unrealized Gain (Loss)	(123.3)	44.1
Equity Securities, FV-NI, Net Gain (Loss)	(122.4)	42.2
Other Invested Assets [Member]
Equity Securities, FV-NI, Gain (Loss)
Equity Securities, FV-NI, Unrealized Gain (Loss)	(15.4)	3.9
Equity Securities, FV-NI, Net Gain (Loss)	$ (15.4)	$ 3.9