Losses And Loss Adjustment Expenses - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012 Cases	Sep. 30, 2011 Cases	Sep. 30, 2012 Cases	Sep. 30, 2011 Cases
Reinsurance Recoverables [Line Items]
Net reduction in ultimate loss and loss adjustment expense liabilities			$ 643,900,000
Net loss reserves acquired			80,500,000
Net reduction in ultimate loss and loss adjustment expense liabilities	62,547,000	50,114,000	141,730,000	88,501,000
Reduction in estimates of net ultimate losses	58,506,000	42,467,000	120,221,000	72,908,000
Reduction in provisions for unallocated loss adjustment expense liabilities	12,579,000	14,113,000	37,092,000	37,433,000
Amortization of fair value adjustment	8,538,000	8,865,000	18,365,000	25,911,000
Net incurred loss development	25,000,000	25,500,000	59,400,000	20,700,000
Advised case reserves	4,400	7,400	4,400	7,400
Reserves with an aggregate value	27,600,000	30,500,000	27,600,000	30,500,000
Aggregate reduction in IBNR reserves	9,700,000	10,700,000	9,700,000	38,700,000
Net change in IBNR	33,528,000	16,961,000	60,780,000	52,237,000
Reduction in provisions for bad debt		$ (2,399,000)	$ (2,782,000)	$ (4,071,000)