Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	$ 55.2	$ 59.3	$ 52.1
Realized losses on fixed maturities and other	(63.3)	(63.6)	(57.2)
Net realized gains on equity securities	128.9	37.4	46.1
Other-than-temporary impairment losses on equity securities	0.0	0.0	(1.7)
Change in unrealized (losses) on equity securities held at the end of the period	(40.8)	(105.1)	0.0
Net realized gains (losses) on equity securities	88.1	(67.7)	44.4
Net realized investment gains (losses)	80.0	(72.0)	39.3
Income tax provision	(16.2)	(11.2)	(12.0)
Net realized investment gains (losses) net of income taxes	63.8	(83.2)	27.3
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	22.2	17.7	25.7
Realized losses on fixed maturities and other	(11.7)	(16.0)	(20.0)
Other Investments [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	33.0	41.4	25.7
Realized losses on fixed maturities and other	(31.3)	(39.5)	(36.2)
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0.0	0.2	0.7
Realized losses on fixed maturities and other	0.0	(0.5)	(0.2)
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized losses on fixed maturities and other	(20.3)	(6.6)	(0.8)
Other-Than-Temporary Impairment Losses on Equity Securities [Member]
Schedule of Investments [Line Items]
Realized losses on fixed maturities and other	$ 0.0	$ (1.0)	$ 0.0