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								12 Months Ended						10 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2012
Revenues
Net premiums earned									$ 4,410,647		$ 4,239,216		$ 3,733,005
Net investment income									459,876		465,664		312,998
Net realized capital gains									247,058		232,119		157,879
Other than temporary impairment losses									(36,294)		(44,047)		(2,907)
Gain on bargain purchase													494,940
Other income									150,522		78,702		57,297
Total revenues	1,302,100	1,351,900	1,291,200	1,286,600	1,362,500	1,189,500	1,196,200	1,223,600	5,231,809		4,971,654		4,753,212
Costs and Expenses
Net loss and loss adjustment expenses									2,494,565		2,479,353		2,630,170
Commissions, brokerage and other underwriting expenses									1,421,306	[1]	1,339,191	[1]	882,502	[1]
Other operating expenses									252,673		164,859		123,700
Corporate administration									47,054		36,111		75,842
Amortization of intangible assets									(5,750)		10,164		253,298
Interest expense									90,052		86,740		68,424
Total costs and expenses									4,299,900		4,116,418		4,033,936
Earnings before income taxes									931,909		855,236		719,276
Income taxes									251,777		225,882		17,032
Net earnings									680,132		629,354		702,244
Reinsurance Segment
Revenues
Net premiums earned															2,915,900
Net investment income															222,000
Net realized capital gains															10,300
Other than temporary impairment losses															0
Gain on bargain purchase															0
Other income															25,500
Total revenues															3,173,700
Costs and Expenses
Net loss and loss adjustment expenses															2,058,100
Commissions, brokerage and other underwriting expenses															591,100
Other operating expenses															67,900
Corporate administration															0
Amortization of intangible assets															249,900
Interest expense															40,500
Total costs and expenses															3,007,500
Earnings before income taxes															166,200
Income taxes															2,400
Net earnings															$ 163,800

[1] Includes amortization associated with deferred acquisition costs of $1,042.0 million, $973.5 million and $578.1 million for the years ended December 31, 2014, 2013 and 2012, respectively.