Investments - Components of Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Net Investment Income [Line Items]
Investment income	$ 19.4	$ 19.4	$ 58.5	$ 57.0
Investment expenses	0.5	0.6	1.5	1.6
Net investment income	18.9	18.8	57.0	55.4
Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment income	16.1	16.2	49.4	48.2
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income	1.6	1.9	4.7	4.7
Cash and Cash Equivalents, and Other [Member]
Net Investment Income [Line Items]
Investment income	$ 1.7	$ 1.3	$ 4.4	$ 4.1