Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	$ 69.0	$ 12.4
Realized losses on fixed maturities and other	(64.2)	(14.8)
Credit losses on fixed maturities	24.7	0.0
Net realized (losses) gains on equity securities	(1.6)	0.7
Change in unrealized (losses) gains on equity securities held at the end of the period	(39.3)	54.2
Net realized (losses) gains on equity securities	(40.9)	54.9
Net realized investment (losses) gains before income taxes	(36.1)	52.5
Income tax benefit (provision)	8.0	(9.7)
Net realized investment (losses) gains net of income taxes	(28.1)	42.8
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	20.8	3.6
Realized losses on fixed maturities and other	(7.7)	(2.4)
Other Investments [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	48.2	8.8
Realized losses on fixed maturities and other	(31.8)	(8.1)
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule of Investments [Line Items]
Credit losses on fixed maturities	$ 24.7	$ 4.3