Components Of Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 286.5	$ 265.9	$ 254.1
Less investment expenses	(9.9)	(7.7)	(6.9)
Net investment income	276.6	258.2	247.2
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Gross investment income	264.2	254.3	248.6
Equity Securities
Gain (Loss) on Investments [Line Items]
Gross investment income	15.3	6.8	4.5
Other Investments
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 7.0	$ 4.8	$ 1.0