Net Investment Income (Detail) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Component of Other Income, Nonoperating [Line Items]
Interest income	$ 100,000,000	$ 108,600,000	$ 113,700,000
Dividend income	37,800,000	23,200,000	15,200,000
Investment expenses	(6,800,000)	(5,500,000)	(7,200,000)
Other investment (loss) income	(300,000)	3,900,000	3,200,000
Net investment income	108,910,000	125,012,000	101,949,000
Investment in Homesite
Component of Other Income, Nonoperating [Line Items]
Equity in losses	(20,200,000)	(3,200,000)	(1,100,000)
ORX Exploration, Inc.
Component of Other Income, Nonoperating [Line Items]
Equity in losses	$ (1,600,000)	$ (2,000,000)	$ (21,900,000)