Tax Effects of Temporary Differences that Give Rise to Significant Portions of Deferred Tax Assets and Deferred Tax Liabilities (Detail) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Deferred tax assets:
Loss and LAE reserves	$ 367,600,000	$ 58,900,000
Minimum tax credit carry forward	179,500,000
Compensation accruals	106,200,000	71,700,000
Unearned premiums	115,100,000	28,800,000
OTTI losses	8,500,000	10,500,000
Foreign tax credit carry forward	41,800,000
State net operating loss carry forward	15,200,000	15,200,000
Other	63,200,000	36,200,000
Gross deferred tax assets before valuation allowance	897,100,000	221,300,000
Valuation allowance	(15,200,000)	(15,200,000)
Gross deferred tax assets	881,900,000	206,100,000
Deferred tax liabilities:
Net unrealized gains on investments	166,000,000	81,900,000
Deferred acquisition costs	100,000,000	25,100,000
Purchase accounting adjustments	144,000,000	3,800,000
Other	22,600,000	14,300,000
Gross deferred tax liabilities	432,600,000	125,100,000
Net deferred tax assets	$ 449,326,000	$ 80,975,000