Results of Transatlantic Since Acquisition Date Included in Consolidated Statements of Earnings and Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended	7 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Mar. 06, 2012 Transatlantic	Sep. 30, 2012 Transatlantic	Sep. 30, 2012 Transatlantic
Revenues
Net premiums earned	$ 1,092,775	$ 190,209	$ 2,622,914	$ 555,067		$ 881,100	$ 2,017,500
Net investment income	90,514	22,050	234,568	82,174		66,300	155,500
Net realized capital gains	12,388	22,706	119,829	63,888		(7,700)	(700)
Other than temporary impairment losses		(2,756)	(2,907)	(2,756)
Gain on bargain purchase			494,940		494,900
Other income	33,843	321	43,434	1,454		24,000	24,700
Total revenues	1,229,520	232,530	3,512,778	699,827		963,700	2,197,000
Costs and Expenses
Net loss and loss adjustment expenses	672,984	121,777	1,587,815	315,418		562,500	1,271,000
Commissions, brokerage and other underwriting expenses	253,756	68,239	592,809	198,899		181,100	377,300
Other operating expenses	47,432	1,757	97,910	18,998		28,600	53,600
Corporate administration	9,304	2,649	67,691	14,030
Amortization of intangible assets	73,356	838	213,393	2,516		72,500	210,800
Interest expense	21,764	4,329	46,584	13,049		12,400	28,100
Total costs and expenses	1,078,596	199,589	2,606,202	562,910		857,100	1,940,800
Earnings before income taxes	150,924	32,941	906,576	136,917		106,600	256,200
Income taxes	25,480	13,783	111,720	31,337		21,100	50,900
Net earnings	$ 125,444	$ 19,158	$ 794,856	$ 105,580		$ 85,500	$ 205,300