Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 246,545		$ 247,822
Reinsurance Recoverables Paid/Unpaid Losses	872,691
Reinsurance Recoverables Total	1,119,236	[1]
Collateral Held	205,440	[2]
Rating	AA
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	50,635
Reinsurance Recoverables Paid/Unpaid Losses	101,575
Reinsurance Recoverables Total	152,210	[1]
Collateral Held	30,795	[2]
Transatlantic Reinsurance Company | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	19,819
Reinsurance Recoverables Paid/Unpaid Losses	75,865
Reinsurance Recoverables Total	95,684	[1]
Collateral Held	6,984	[2]
Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	21,782
Reinsurance Recoverables Paid/Unpaid Losses	70,793
Reinsurance Recoverables Total	92,575	[1]
Collateral Held	7,706	[2]
Munich Reinsurance America Inc. | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	5,505
Reinsurance Recoverables Paid/Unpaid Losses	68,798
Reinsurance Recoverables Total	74,303	[1]
Collateral Held	4,510	[2]
Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	7,690
Reinsurance Recoverables Paid/Unpaid Losses	63,942
Reinsurance Recoverables Total	71,632	[1]
Collateral Held	4,759	[2]
Lloyd Syndicate #2003 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	6,704
Reinsurance Recoverables Paid/Unpaid Losses	40,585
Reinsurance Recoverables Total	47,289	[1]
Collateral Held	5,977	[2]
Allied World Reinsurance | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	13,380
Reinsurance Recoverables Paid/Unpaid Losses	33,043
Reinsurance Recoverables Total	46,423	[1]
Collateral Held	3,129	[2]
Partner Reinsurance Europe | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	9,483
Reinsurance Recoverables Paid/Unpaid Losses	29,208
Reinsurance Recoverables Total	38,691	[1]
Collateral Held	16,623	[2]
Scor Global P&C SE | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	9,690
Reinsurance Recoverables Paid/Unpaid Losses	22,199
Reinsurance Recoverables Total	31,889	[1]
Collateral Held	6,724	[2]
Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,368
Reinsurance Recoverables Paid/Unpaid Losses	14,766
Reinsurance Recoverables Total	26,134	[1]
Collateral Held	10,689	[2]
Atlantic Specialty Insurance | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	6,741
Reinsurance Recoverables Paid/Unpaid Losses	15,944
Reinsurance Recoverables Total	22,685	[1]
Collateral Held	839	[2]
Tower Insurance Company | AM Best, B Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Paid/Unpaid Losses	21,738
Reinsurance Recoverables Total	21,738	[1]
Collateral Held	1,803	[2]
QBE Reinsurance Corp | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	6,666
Reinsurance Recoverables Paid/Unpaid Losses	12,827
Reinsurance Recoverables Total	19,493	[1]
Collateral Held	1,043	[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	10,108
Reinsurance Recoverables Paid/Unpaid Losses	7,884
Reinsurance Recoverables Total	17,992	[1]
Collateral Held	4,152	[2]
Validus Reinsurance Ltd | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,316
Reinsurance Recoverables Paid/Unpaid Losses	15,198
Reinsurance Recoverables Total	17,514	[1]
Collateral Held	12,140	[2]
Ironshore Indemnity Inc. | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	7,224
Reinsurance Recoverables Paid/Unpaid Losses	10,248
Reinsurance Recoverables Total	17,472	[1]
Collateral Held	9,273	[2]
Lloyd Syndicate #4000 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	96
Reinsurance Recoverables Paid/Unpaid Losses	16,127
Reinsurance Recoverables Total	16,223	[1]
Collateral Held	111	[2]
Odyssey American Reinsurance Corporation | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	3,420
Reinsurance Recoverables Paid/Unpaid Losses	12,425
Reinsurance Recoverables Total	15,845	[1]
Collateral Held	1,545	[2]
Berkley Insurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,096
Reinsurance Recoverables Paid/Unpaid Losses	11,846
Reinsurance Recoverables Total	13,942	[1]
AXIS Re Europe | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	4,108
Reinsurance Recoverables Paid/Unpaid Losses	9,453
Reinsurance Recoverables Total	13,561	[1]
Collateral Held	3,724	[2]
Insurance Companies
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	198,831
Reinsurance Recoverables Paid/Unpaid Losses	654,464
Reinsurance Recoverables Total	853,295	[1]
Collateral Held	132,526	[2]
All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	47,714
Reinsurance Recoverables Paid/Unpaid Losses	218,227
Reinsurance Recoverables Total	265,941	[1]
Collateral Held	$ 72,914	[2]
AM Best | National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ 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 | Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ 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 | Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's, AA- 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 | Allied World Reinsurance | 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 | Atlantic Specialty Insurance | 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 | QBE Reinsurance Corp | AM Best, A Rating | Standard & Poor's, A+ Rating
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 | Validus Reinsurance Ltd | 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 | Lloyd Syndicate #4000 | 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 | Berkley Insurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
AM Best | AXIS Re Europe | 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 | Transatlantic Reinsurance Company | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
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 | Munich Reinsurance America Inc. | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating	AA-
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 | Lloyd Syndicate #2003 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
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 | 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 | Atlantic Specialty Insurance | 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 | QBE Reinsurance Corp | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
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 | Validus Reinsurance Ltd | 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 | Lloyd Syndicate #4000 | 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 | Berkley Insurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Standard & Poor's | AXIS Re Europe | 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 $205.4 million consists of $145.9 million in ceded balances payable, $53.4 million in letters of credit, and $6.1 million of other balances held by the Company's Insurance Companies and Lloyd's Operations.