Investments - Components of Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Net Investment Income [Line Items]
Investment income	$ 21.0	$ 20.2	$ 39.1	$ 37.6
Investment expenses	0.5	0.5	1.0	1.0
Net investment income	20.5	19.7	38.1	36.6
Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment income	17.9	17.2	33.3	32.0
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income	1.7	1.6	3.1	2.8
Cash and Cash Equivalents, and Other [Member]
Net Investment Income [Line Items]
Investment income	$ 1.4	$ 1.4	$ 2.7	$ 2.8