Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	$ 59.3	$ 52.1	$ 70.7
Realized losses on fixed maturities and other	(63.6)	(57.2)	(83.9)
Net realized gains on equity securities	37.4	46.1	47.8
Other-than-temporary impairment losses on equity securities	0.0	(1.7)	(8.5)
Change in unrealized (losses) gains on equity securities held at the end of the period	(105.1)	0.0	0.0
Net realized (losses) gains on equity securities	(67.7)	44.4	39.3
Net realized investment and other (losses) gains	(72.0)	39.3	26.1
Income tax expense	(11.2)	(12.0)	(11.6)
After tax	(83.2)	27.3	14.5
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	17.7	25.7	21.5
Realized losses on fixed maturities and other	(16.0)	(20.0)	(35.9)
Other Investments [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	41.4	25.7	47.5
Realized losses on fixed maturities and other	(39.5)	(36.2)	(46.0)
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0.2	0.7	0.5
Realized losses on fixed maturities and other	(0.5)	(0.2)	(0.3)
Other Assets [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0.0	0.0	1.2
Realized losses on fixed maturities and other	0.0	0.0	0.0
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized losses on fixed maturities and other	(6.6)	(0.8)	(1.7)
Other-Than-Temporary Impairment Losses on Equity Securities [Member]
Schedule of Investments [Line Items]
Realized losses on fixed maturities and other	$ (1.0)	$ 0.0	$ 0.0