Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 237,094		$ 247,822
Reinsurance Recoverables Paid/Unpaid Losses	899,909
Reinsurance Recoverables Total	1,137,003	[1]
Collateral Held	217,161	[2]
Rating	AA
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	32,242
Reinsurance Recoverables Paid/Unpaid Losses	121,079
Reinsurance Recoverables Total	153,321	[1]
Collateral Held	25,254	[2]
Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	21,240
Reinsurance Recoverables Paid/Unpaid Losses	75,201
Reinsurance Recoverables Total	96,441	[1]
Collateral Held	7,014	[2]
Transatlantic Reinsurance Company | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	13,544
Reinsurance Recoverables Paid/Unpaid Losses	77,611
Reinsurance Recoverables Total	91,155	[1]
Collateral Held	4,616	[2]
Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	19,776
Reinsurance Recoverables Paid/Unpaid Losses	63,823
Reinsurance Recoverables Total	83,599	[1]
Collateral Held	13,336	[2]
Munich Reinsurance America Inc. | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	9,810
Reinsurance Recoverables Paid/Unpaid Losses	65,695
Reinsurance Recoverables Total	75,505	[1]
Collateral Held	6,074	[2]
Allied World Reinsurance | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	9,804
Reinsurance Recoverables Paid/Unpaid Losses	36,819
Reinsurance Recoverables Total	46,623	[1]
Collateral Held	1,834	[2]
Lloyd Syndicate #2003 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	3,869
Reinsurance Recoverables Paid/Unpaid Losses	39,976
Reinsurance Recoverables Total	43,845	[1]
Collateral Held	5,030	[2]
Partner Reinsurance Europe | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	10,123
Reinsurance Recoverables Paid/Unpaid Losses	29,508
Reinsurance Recoverables Total	39,631	[1]
Collateral Held	19,146	[2]
Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,848
Reinsurance Recoverables Paid/Unpaid Losses	20,187
Reinsurance Recoverables Total	32,035	[1]
Collateral Held	11,298	[2]
Scor Global P&C SE | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	10,030
Reinsurance Recoverables Paid/Unpaid Losses	19,711
Reinsurance Recoverables Total	29,741	[1]
Collateral Held	7,483	[2]
Tower Insurance Company | AM Best, B- Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Paid/Unpaid Losses	20,923
Reinsurance Recoverables Total	20,923	[1]
Collateral Held	2,408	[2]
Ironshore Indemnity Inc. | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	7,242
Reinsurance Recoverables Paid/Unpaid Losses	13,313
Reinsurance Recoverables Total	20,555	[1]
Collateral Held	9,793	[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	10,900
Reinsurance Recoverables Paid/Unpaid Losses	9,562
Reinsurance Recoverables Total	20,462	[1]
Collateral Held	2,717	[2]
Atlantic Specialty Insurance | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	3,085
Reinsurance Recoverables Paid/Unpaid Losses	16,714
Reinsurance Recoverables Total	19,799	[1]
QBE Reinsurance Corp | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	3,466
Reinsurance Recoverables Paid/Unpaid Losses	15,322
Reinsurance Recoverables Total	18,788	[1]
Aspen Insurance UK Ltd | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	8,100
Reinsurance Recoverables Paid/Unpaid Losses	9,475
Reinsurance Recoverables Total	17,575	[1]
Collateral Held	4,944	[2]
Validus Reinsurance Ltd | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,164
Reinsurance Recoverables Paid/Unpaid Losses	14,887
Reinsurance Recoverables Total	17,051	[1]
Collateral Held	13,456	[2]
Odyssey American Reinsurance Corporation | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	3,106
Reinsurance Recoverables Paid/Unpaid Losses	12,580
Reinsurance Recoverables Total	15,686	[1]
Collateral Held	1,916	[2]
Endurance Reinsurance Corporation | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	5,529
Reinsurance Recoverables Paid/Unpaid Losses	9,625
Reinsurance Recoverables Total	15,154	[1]
Collateral Held	1,814	[2]
Lloyd Syndicate #4000 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	19
Reinsurance Recoverables Paid/Unpaid Losses	14,579
Reinsurance Recoverables Total	14,598	[1]
Collateral Held	98	[2]
Insurance Companies
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	185,897
Reinsurance Recoverables Paid/Unpaid Losses	686,590
Reinsurance Recoverables Total	872,487	[1]
Collateral Held	138,231	[2]
All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	51,197
Reinsurance Recoverables Paid/Unpaid Losses	213,319
Reinsurance Recoverables Total	264,516	[1]
Collateral Held	$ 78,930	[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 | Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
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 | Tower Insurance Company | AM Best, B- Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	B-
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 | Atlantic Specialty Insurance | AM Best, A Rating | Standard & Poor's, A- 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 | Aspen Insurance UK Ltd | 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 | Endurance Reinsurance Corporation | 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
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 | Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	NR
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 | Tower Insurance Company | AM Best, B- Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	NR
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 | Atlantic Specialty Insurance | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Rating	A-
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 | Aspen Insurance UK Ltd | 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 | Endurance Reinsurance Corporation | 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+

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

[2] Collateral of $217.2 million consists of $157.9 million in ceded balances payable, $53.4 million in letters of credit, and $5.9 million of other balances held by the Company's Insurance Companies and Lloyd's Operations.