Investments - Components of Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Investment Income [Line Items]
Investment income	$ 21.8	$ 21.8	$ 41.4	$ 42.1
Investment expenses	0.1	0.3	0.3	0.7
Net investment income	21.7	21.5	41.1	41.4
Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment income	17.3	16.9	32.4	33.0
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income	3.1	3.1	5.9	5.6
Cash and Cash Equivalents, and Other [Member]
Net Investment Income [Line Items]
Investment income	$ 1.4	$ 1.8	$ 3.1	$ 3.5