Lists of 10 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 226,723	$ 228,569
Reinsurance Recoverables Paid/Unpaid Losses	[1]	938,448
Reinsurance Recoverables Total		1,165,171
Collateral Held		205,917
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		5,338
Reinsurance Recoverables Paid/Unpaid Losses	[1]	61,540
Reinsurance Recoverables Total		66,878
Collateral Held		$ 8,320
National Indemnity Company | AM Best, A++ Rating | AM Best | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Rating		A++
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Rating		AA+
Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 34,809
Reinsurance Recoverables Paid/Unpaid Losses	[1]	106,334
Reinsurance Recoverables Total		141,143
Collateral Held		$ 1,547
Everest Reinsurance Company | AM Best, A+ Rating | AM Best | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A+
Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A+
Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 13,514
Reinsurance Recoverables Paid/Unpaid Losses	[1]	64,457
Reinsurance Recoverables Total		77,971
Collateral Held		$ 4,223
Swiss Reinsurance America Corporation | AM Best, A+ Rating | AM Best | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		A+
Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		AA-
Transatlantic Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 7,597
Reinsurance Recoverables Paid/Unpaid Losses	[1]	48,328
Reinsurance Recoverables Total		55,925
Collateral Held		$ 2,541
Transatlantic Reinsurance Company | AM Best, A+ Rating | AM Best | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A+
Transatlantic Reinsurance Company | AM Best, A+ Rating | Standard & Poor's | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A+
Munich Reinsurance America Inc | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 10,932
Reinsurance Recoverables Paid/Unpaid Losses	[1]	41,369
Reinsurance Recoverables Total		52,301
Collateral Held		$ 6,054
Munich Reinsurance America Inc | AM Best, A+ Rating | AM Best | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		A+
Munich Reinsurance America Inc | AM Best, A+ Rating | Standard & Poor's | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		AA-
Allied World Reinsurance | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 7,303
Reinsurance Recoverables Paid/Unpaid Losses	[1]	30,439
Reinsurance Recoverables Total		37,742
Collateral Held		$ 1,757
Allied World Reinsurance | AM Best, A Rating | AM Best | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Rating		A
Allied World Reinsurance | AM Best, A Rating | Standard & Poor's | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Rating		A-
Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 16,491
Reinsurance Recoverables Paid/Unpaid Losses	[1]	34,445
Reinsurance Recoverables Total		50,936
Collateral Held		$ 18,120
Employers Mutual Casualty Company | AM Best, A Rating | AM Best | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating		A
Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating		NR
Aspen Insurance UK Ltd. | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 11,801
Reinsurance Recoverables Paid/Unpaid Losses	[1]	31,508
Reinsurance Recoverables Total		43,309
Collateral Held		$ 6,446
Aspen Insurance UK Ltd. | AM Best, A Rating | AM Best | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating		A
Aspen Insurance UK Ltd. | AM Best, A Rating | Standard & Poor's | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating		A
XL Reinsurance America Inc. | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 7,240
Reinsurance Recoverables Paid/Unpaid Losses	[1]	21,250
Reinsurance Recoverables Total		28,490
Collateral Held		$ 2,761
XL Reinsurance America Inc. | AM Best, A+ Rating | AM Best | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		A+
XL Reinsurance America Inc. | AM Best, A+ Rating | Standard & Poor's | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		AA-
Ace Property & Casualty Insurance Company | AM Best, A++ Rating | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 1,591
Reinsurance Recoverables Paid/Unpaid Losses	[1]	24,148
Reinsurance Recoverables Total		25,739
Collateral Held		$ 63
Ace Property & Casualty Insurance Company | AM Best, A++ Rating | AM Best | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Rating		A++
Ace Property & Casualty Insurance Company | AM Best, A++ Rating | Standard & Poor's | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Rating		AA
Insurance Companies
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 116,616
Reinsurance Recoverables Paid/Unpaid Losses	[1]	463,818
Reinsurance Recoverables Total		580,434
Collateral Held		51,832
All Others
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		110,107
Reinsurance Recoverables Paid/Unpaid Losses	[1]	474,630
Reinsurance Recoverables Total		584,737
Collateral Held		$ 154,085

[1] Net of reserve for uncollectible reinsurance of approximately $13.4 million.