Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 237,851		$ 247,822
Reinsurance Recoverables Paid/Unpaid Losses	902,845
Reinsurance Recoverables Total	1,140,696	[1],[2],[3]
Collateral Held	201,976
Rating	AA
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	25,202
Reinsurance Recoverables Paid/Unpaid Losses	117,562
Reinsurance Recoverables Total	142,764	[1]
Collateral Held	22,069
Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	21,573
Reinsurance Recoverables Paid/Unpaid Losses	75,063
Reinsurance Recoverables Total	96,636	[1]
Collateral Held	7,326
Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	22,815
Reinsurance Recoverables Paid/Unpaid Losses	73,305
Reinsurance Recoverables Total	96,120	[1]
Collateral Held	14,587
Transatlantic Reinsurance Company | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,916
Reinsurance Recoverables Paid/Unpaid Losses	74,072
Reinsurance Recoverables Total	85,988	[1]
Collateral Held	4,038
Munich Reinsurance America Inc. | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,366
Reinsurance Recoverables Paid/Unpaid Losses	58,768
Reinsurance Recoverables Total	70,134	[1]
Collateral Held	5,539
Allied World Reinsurance | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	9,048
Reinsurance Recoverables Paid/Unpaid Losses	37,088
Reinsurance Recoverables Total	46,136	[1]
Collateral Held	1,666
Lloyd Syndicate #2003 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	4,399
Reinsurance Recoverables Paid/Unpaid Losses	35,123
Reinsurance Recoverables Total	39,522	[1]
Collateral Held	5,191
Partner Reinsurance Europe | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	10,986
Reinsurance Recoverables Paid/Unpaid Losses	25,409
Reinsurance Recoverables Total	36,395	[1]
Collateral Held	16,052
Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,928
Reinsurance Recoverables Paid/Unpaid Losses	21,851
Reinsurance Recoverables Total	33,779	[1]
Collateral Held	10,935
Scor Global P&C SE | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	10,190
Reinsurance Recoverables Paid/Unpaid Losses	17,572
Reinsurance Recoverables Total	27,762	[1]
Collateral Held	5,558
Ace Property and Casualty Insurance Company | AM Best, A++ Rating | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,165
Reinsurance Recoverables Paid/Unpaid Losses	12,741
Reinsurance Recoverables Total	23,906	[1]
Collateral Held	2,907
Tower Insurance Company | AM Best, A- Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Paid/Unpaid Losses	21,509
Reinsurance Recoverables Total	21,509	[1]
Collateral Held	2,455
Aspen Insurance UK Ltd. | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	8,928
Reinsurance Recoverables Paid/Unpaid Losses	11,227
Reinsurance Recoverables Total	20,155	[1]
Collateral Held	4,869
Ironshore Indemnity Inc. | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	6,234
Reinsurance Recoverables Paid/Unpaid Losses	13,395
Reinsurance Recoverables Total	19,629	[1]
Collateral Held	8,645
Validus Reinsurance Ltd | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,020
Reinsurance Recoverables Paid/Unpaid Losses	16,873
Reinsurance Recoverables Total	18,893	[1]
Collateral Held	10,975
Atlantic Specialty Insurance | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,542
Reinsurance Recoverables Paid/Unpaid Losses	15,812
Reinsurance Recoverables Total	18,354	[1]
QBE Reinsurance Corp | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,636
Reinsurance Recoverables Paid/Unpaid Losses	15,539
Reinsurance Recoverables Total	18,175	[1]
National Union Fire Ins. | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	8,067
Reinsurance Recoverables Paid/Unpaid Losses	8,459
Reinsurance Recoverables Total	16,526	[1]
Collateral Held	6,158
Endurance Reinsurance Corporation | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	5,695
Reinsurance Recoverables Paid/Unpaid Losses	9,936
Reinsurance Recoverables Total	15,631	[1]
Collateral Held	1,337
Odyssey American Reinsurance Corporation | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	3,506
Reinsurance Recoverables Paid/Unpaid Losses	11,650
Reinsurance Recoverables Total	15,156	[1]
Collateral Held	1,604
Insurance Companies
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	190,216
Reinsurance Recoverables Paid/Unpaid Losses	672,954
Reinsurance Recoverables Total	863,170	[1]
Collateral Held	131,911
All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	47,635
Reinsurance Recoverables Paid/Unpaid Losses	229,891
Reinsurance Recoverables Total	277,526	[1]
Collateral Held	$ 70,065
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 | Swiss Reinsurance America Corporation | 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 | 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 | Ace Property and Casualty Insurance Company | AM Best, A++ Rating | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Rating	A++
AM Best | Tower Insurance Company | AM Best, A- Rating | Standard & Poor's, Not Rated
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 | Ironshore Indemnity Inc. | AM Best, A Rating | Standard & Poor's, Not Rated
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 | 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 | National Union Fire Ins. | 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 | Odyssey American 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 | Swiss Reinsurance America Corporation | 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 | 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 | 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 | Tower Insurance Company | AM Best, A- Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	NR
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 | Ironshore Indemnity Inc. | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	NR
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 | 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 | National Union Fire Ins. | 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 | Odyssey American 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] When an A.M. Best rating is unavailable, the equivalent S&P rating is used.

[3] The carrying value is comprised of prepaid reinsurance premium as well as reinsurance recoverables on paid and unpaid losses which are net of the reserve for uncollectible reinsurance.