Investment Income and Gains and Losses - Additional Information (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]
Non-accruals of fixed maturity securities	$ 2.5	$ 2.3	$ 2.3
Net other-than-temporary impairment losses on securities recognized in earnings	7.8	6.9	13.9
Other-than-temporary impairments	7.3	4.9	9.4
Amount transferred from unrealized losses in accumulated other comprehensive income	0.5	2.0	4.5
OTTI related to fixed maturity securities intended to sell	7.8	4.6	3.3
OTTI related to common stocks	5.4	1.4	1.9
OTTI related to estimated credit losses on fixed maturity securities	1.2	0.9	4.3
OTTI related to certain low-income housing tax credit limited partnerships			4.4
Below Investment Grade | Corporate Bond Securities
Gain (Loss) on Investments [Line Items]
OTTI related to fixed maturity securities intended to sell	$ 1.2