Investments - Components of Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net Investment Income [Line Items]
Investment income	$ 21.1	$ 19.0	$ 63.2	$ 57.4
Investment expenses	0.3	0.3	1.0	0.9
Net investment income	20.8	18.7	62.2	56.5
Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment income	16.4	15.4	49.4	47.1
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income	3.0	2.0	8.6	5.7
Cash and Cash Equivalents, and Other [Member]
Net Investment Income [Line Items]
Investment income	$ 1.7	$ 1.6	$ 5.2	$ 4.6