Investments - Schedule of Investment Income and Expenses (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Amortized Cost And Fair Value Debt Securities [Abstract]
Interest and dividends on fixed maturities	$ 76.3	$ 87.7	$ 105.9
Dividends on equity securities	15.8	17.7	15.2
Interest on short-term and other investments	5.3	5.0	7.3
Other	(1.0)	(1.0)	(0.4)
Investment income	96.4	109.4	128.0
Investment expenses	(9.8)	(9.4)	(9.2)
Net investment income	$ 86.6	$ 100.0	$ 118.8