Investments - Components of Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net Investment Income [Line Items]
Investment income	$ 19.4	$ 19.5	$ 38.4	$ 37.3
Investment expenses	0.3	0.4	0.6	0.8
Net investment income	19.1	19.1	37.8	36.5
Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment income	15.9	16.7	31.7	31.6
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income	2.0	1.5	3.7	3.0
Cash and Cash Equivalents, and Other [Member]
Net Investment Income [Line Items]
Investment income	$ 1.5	$ 1.3	$ 3.0	$ 2.7