Net Investment Income (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Interest income	$ 88,200,000	$ 25,200,000	$ 218,200,000	$ 76,200,000
Dividend income	8,300,000	9,600,000	14,500,000	29,800,000
Investment expenses	(3,700,000)	(1,500,000)	(11,600,000)	(4,800,000)
Other investment income	1,500,000	(400,000)	8,200,000	1,100,000
Total	90,514,000	22,050,000	234,568,000	82,174,000
Investment in Homesite
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity income (losses)	(2,900,000)	(9,600,000)	9,500,000	(16,700,000)
ORX Exploration, Incorporated
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity income (losses)	$ (900,000)	$ (1,200,000)	$ (4,200,000)	$ (3,400,000)