Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	$ 11.2	$ 12.2	$ 34.9	$ 50.7
Realized losses on fixed maturities and other	(15.0)	(11.6)	(42.3)	(50.5)
Net realized gains on equity securities	6.4	9.1	7.7	30.9
Change in unrealized (losses) gains on equity securities held at the end of the period	(8.8)	4.5	58.0	(22.1)
Net realized (losses) gains on equity securities	(2.4)	13.6	65.7	8.8
Net realized investment (losses) gains before income taxes	(6.2)	14.2	58.3	9.0
Income tax benefit (provision)	0.3	(3.3)	(11.9)	(2.8)
Net realized investment (losses) gains net of income taxes	(5.9)	10.9	46.4	6.2
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	3.7	4.6	13.0	15.8
Realized losses on fixed maturities and other	(1.6)	(2.8)	(8.7)	(14.2)
Other Investments [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	7.5	7.4	21.9	34.7
Realized losses on fixed maturities and other	(8.4)	(8.8)	(21.8)	(34.4)
Other Assets and Short-term Investments [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0.0	0.2	0.0	0.2
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized losses on fixed maturities and other	$ (5.0)	$ 0.0	$ (11.8)	$ (1.9)