Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014		Dec. 31, 2013
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 246,699		$ 247,822
Reinsurance Recoverables Paid/Unpaid Losses	884,055
Reinsurance Recoverables Total	1,130,754	[1]
Collateral Held	218,667	[2]
Rating	AA
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	40,640
Reinsurance Recoverables Paid/Unpaid Losses	114,646
Reinsurance Recoverables Total	155,286	[1]
Collateral Held	24,453	[2]
Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	21,201
Reinsurance Recoverables Paid/Unpaid Losses	72,994
Reinsurance Recoverables Total	94,195	[1]
Collateral Held	7,206	[2]
Transatlantic Reinsurance Company | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	16,038
Reinsurance Recoverables Paid/Unpaid Losses	75,395
Reinsurance Recoverables Total	91,433	[1]
Collateral Held	4,930	[2]
Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	15,471
Reinsurance Recoverables Paid/Unpaid Losses	65,276
Reinsurance Recoverables Total	80,747	[1]
Collateral Held	13,413	[2]
Munich Reinsurance America Inc. | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	8,095
Reinsurance Recoverables Paid/Unpaid Losses	66,415
Reinsurance Recoverables Total	74,510	[1]
Collateral Held	6,269	[2]
Allied World Reinsurance | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,159
Reinsurance Recoverables Paid/Unpaid Losses	35,260
Reinsurance Recoverables Total	46,419	[1]
Collateral Held	1,924	[2]
Lloyd Syndicate #2003 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	4,911
Reinsurance Recoverables Paid/Unpaid Losses	39,768
Reinsurance Recoverables Total	44,679	[1]
Collateral Held	5,501	[2]
Partner Reinsurance Europe | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	9,945
Reinsurance Recoverables Paid/Unpaid Losses	28,708
Reinsurance Recoverables Total	38,653	[1]
Collateral Held	18,380	[2]
Scor Global P&C SE | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	10,652
Reinsurance Recoverables Paid/Unpaid Losses	20,547
Reinsurance Recoverables Total	31,199	[1]
Collateral Held	8,061	[2]
Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	12,432
Reinsurance Recoverables Paid/Unpaid Losses	18,628
Reinsurance Recoverables Total	31,060	[1]
Collateral Held	11,306	[2]
Tower Insurance Company | AM Best, C++ Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Paid/Unpaid Losses	20,916
Reinsurance Recoverables Total	20,916	[1]
Collateral Held	1,922	[2]
Atlantic Specialty Insurance | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	4,494
Reinsurance Recoverables Paid/Unpaid Losses	16,260
Reinsurance Recoverables Total	20,754	[1]
Ironshore Indemnity Inc. | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	7,743
Reinsurance Recoverables Paid/Unpaid Losses	12,789
Reinsurance Recoverables Total	20,532	[1]
Collateral Held	9,643	[2]
Ace Property and Casualty Insurance Company | AM Best, A++ Rating | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,127
Reinsurance Recoverables Paid/Unpaid Losses	8,020
Reinsurance Recoverables Total	19,147	[1]
Collateral Held	2,988	[2]
QBE Reinsurance Corp | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	4,696
Reinsurance Recoverables Paid/Unpaid Losses	14,139
Reinsurance Recoverables Total	18,835	[1]
Validus Reinsurance Ltd | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,297
Reinsurance Recoverables Paid/Unpaid Losses	16,030
Reinsurance Recoverables Total	18,327	[1]
Collateral Held	13,675	[2]
Odyssey American Reinsurance Corporation | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	3,800
Reinsurance Recoverables Paid/Unpaid Losses	11,889
Reinsurance Recoverables Total	15,689	[1]
Collateral Held	1,726	[2]
Aspen Insurance UK Ltd | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	7,179
Reinsurance Recoverables Paid/Unpaid Losses	8,428
Reinsurance Recoverables Total	15,607	[1]
Collateral Held	5,020	[2]
Lloyd Syndicate #4000 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	51
Reinsurance Recoverables Paid/Unpaid Losses	14,181
Reinsurance Recoverables Total	14,232	[1]
Collateral Held	92	[2]
Endurance Reinsurance Corporation | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	5,432
Reinsurance Recoverables Paid/Unpaid Losses	8,411
Reinsurance Recoverables Total	13,843	[1]
Collateral Held	2,047	[2]
Insurance Companies
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	197,363
Reinsurance Recoverables Paid/Unpaid Losses	668,700
Reinsurance Recoverables Total	866,063	[1]
Collateral Held	138,556	[2]
All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	49,336
Reinsurance Recoverables Paid/Unpaid Losses	215,355
Reinsurance Recoverables Total	264,691	[1]
Collateral Held	$ 80,111	[2]
AM Best | National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Rating	A++
AM Best | Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
AM Best | Transatlantic Reinsurance Company | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating	A+
AM Best | Munich Reinsurance America Inc. | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating	A+
AM Best | Allied World Reinsurance | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Lloyd Syndicate #2003 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Partner Reinsurance Europe | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
AM Best | Scor Global P&C SE | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Tower Insurance Company | AM Best, C++ Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	C++
AM Best | Atlantic Specialty Insurance | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Ironshore Indemnity Inc. | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Ace Property and Casualty Insurance Company | AM Best, A++ Rating | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Rating	A++
AM Best | QBE Reinsurance Corp | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Validus Reinsurance Ltd | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Odyssey American Reinsurance Corporation | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Aspen Insurance UK Ltd | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Lloyd Syndicate #4000 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
AM Best | Endurance Reinsurance Corporation | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A
Standard & Poor's | National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Rating	AA+
Standard & Poor's | Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Standard & Poor's | Transatlantic Reinsurance Company | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Standard & Poor's | Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating	AA-
Standard & Poor's | Munich Reinsurance America Inc. | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating	AA-
Standard & Poor's | Allied World Reinsurance | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A
Standard & Poor's | Lloyd Syndicate #2003 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Standard & Poor's | Partner Reinsurance Europe | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Standard & Poor's | Scor Global P&C SE | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Standard & Poor's | Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	NR
Standard & Poor's | Tower Insurance Company | AM Best, C++ Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	NR
Standard & Poor's | Atlantic Specialty Insurance | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Rating	A-
Standard & Poor's | Ironshore Indemnity Inc. | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	NR
Standard & Poor's | Ace Property and Casualty Insurance Company | AM Best, A++ Rating | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Rating	AA
Standard & Poor's | QBE Reinsurance Corp | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Standard & Poor's | Validus Reinsurance Ltd | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A
Standard & Poor's | Odyssey American Reinsurance Corporation | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Rating	A-
Standard & Poor's | Aspen Insurance UK Ltd | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A
Standard & Poor's | Lloyd Syndicate #4000 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Standard & Poor's | Endurance Reinsurance Corporation | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A

[1] Net of reserve for uncollectible reinsurance of approximately $11.3 million.

[2] Collateral of $218.7 million consists of $159.3 million in ceded balances payable, $53.4 million in letters of credit, and $6.0 million of other balances held by the Company's Insurance Companies and Lloyd's Operations.