Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	$ 19.3	$ 22.6	$ 38.5	$ 51.6
Realized losses on fixed maturities and other	(23.7)	(18.1)	(38.9)	(32.5)
Net realized gains on equity securities sold during the period	10.6	0.0	21.8	0.0
Change in unrealized gains (losses) on equity securities held at the end of the period	4.3	0.0	(26.6)	0.0
Net realized gains (losses) on equity securities	14.9	0.0	(4.8)	0.0
Net realized investment gains (losses) before income taxes	10.5	4.5	(5.2)	19.1
Income tax (expense) benefit	(2.4)	(1.3)	0.5	(5.7)
Net realized investment gains (losses), net of income taxes	8.1	3.2	(4.7)	13.4
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	3.7	5.5	11.2	12.6
Realized losses on fixed maturities and other	(6.5)	(5.0)	(11.4)	(11.2)
Equity Securities [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0.0	12.0	0.0	27.7
Realized losses on fixed maturities and other	0.0	(1.8)	0.0	(2.5)
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	15.6	5.1	27.3	11.3
Realized losses on fixed maturities and other	(16.3)	(10.2)	(25.6)	(17.3)
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized losses on fixed maturities and other	(0.9)	0.0	(1.9)	0.0
Other-Than-Temporary Impairment Losses on Equity Securities [Member]
Schedule of Investments [Line Items]
Realized losses on fixed maturities and other	$ 0.0	$ (1.1)	$ 0.0	$ (1.5)