Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	$ 11.2	$ 19.3	$ 23.7		$ 38.5
Realized losses on fixed maturities and other	(12.4)	(23.7)	(27.3)		(38.9)
Net realized gains on equity securities	0.6	10.6	1.3		21.8
Change in unrealized gains (losses) on equity securities held at the end of the period	12.6	4.3	66.8	$ (26.6)	(26.6)
Net realized gains (losses) on equity securities	13.2	14.9	68.1		(4.8)
Net realized investment gains (losses) before income taxes	12.0	10.5	64.5	$ (5.2)	(5.2)
Income tax (provision) benefit	(2.6)	(2.4)	(12.2)		0.5
Net realized investment gains (losses), net of income taxes	9.4	8.1	52.3		(4.7)
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	5.7	3.7	9.4		11.2
Realized losses on fixed maturities and other	(4.7)	(6.5)	(7.2)		(11.4)
Equity Securities [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	5.5	15.6	14.3		27.3
Realized losses on fixed maturities and other	(5.2)	(16.3)	(13.3)		(25.6)
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized losses on fixed maturities and other	$ (2.5)	$ (0.9)	$ (6.8)		$ (1.9)