Lists of 10 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 213,377	$ 232,588
Reinsurance Recoverables Paid/Unpaid Losses	[1]	861,858
Reinsurance Recoverables Total		1,075,235
Collateral Held		151,708
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		7,467
Reinsurance Recoverables Paid/Unpaid Losses	[1]	98,759
Reinsurance Recoverables Total		106,226
Collateral Held		$ 3,183
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		$ 25,321
Reinsurance Recoverables Paid/Unpaid Losses	[1]	82,154
Reinsurance Recoverables Total		107,475
Collateral Held		$ 6,571
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		$ 22,442
Reinsurance Recoverables Paid/Unpaid Losses	[1]	77,199
Reinsurance Recoverables Total		99,641
Collateral Held		$ 6,440
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		$ 7,669
Reinsurance Recoverables Paid/Unpaid Losses	[1]	53,347
Reinsurance Recoverables Total		61,016
Collateral Held		$ 2,494
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		$ 8,959
Reinsurance Recoverables Paid/Unpaid Losses	[1]	50,612
Reinsurance Recoverables Total		59,571
Collateral Held		$ 3,477
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		$ 6,319
Reinsurance Recoverables Paid/Unpaid Losses	[1]	35,931
Reinsurance Recoverables Total		42,250
Collateral Held		$ 1,575
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		$ 8,856
Reinsurance Recoverables Paid/Unpaid Losses	[1]	32,687
Reinsurance Recoverables Total		41,543
Collateral Held		$ 5,551
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		$ 11,281
Reinsurance Recoverables Paid/Unpaid Losses	[1]	22,737
Reinsurance Recoverables Total		34,018
Collateral Held		$ 5,452
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
Ace Property and Casualty Insurance Company | AM Best, A++ Rating | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 10,192
Reinsurance Recoverables Paid/Unpaid Losses	[1]	22,352
Reinsurance Recoverables Total		32,544
Collateral Held		$ 2,722
Ace Property and Casualty Insurance Company | AM Best, A++ Rating | AM Best | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Rating		A++
Ace Property and Casualty Insurance Company | AM Best, A++ Rating | Standard & Poor's | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Rating		AA
Partner Reinsurance Europe | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 6,868
Reinsurance Recoverables Paid/Unpaid Losses	[1]	20,681
Reinsurance Recoverables Total		27,549
Collateral Held		$ 25,045
Partner Reinsurance Europe | AM Best, A Rating | AM Best | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A
Partner Reinsurance Europe | AM Best, A Rating | Standard & Poor's | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A+
Insurance Companies
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 115,374
Reinsurance Recoverables Paid/Unpaid Losses	[1]	496,459
Reinsurance Recoverables Total		611,833
Collateral Held		62,510
All Others
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		98,003
Reinsurance Recoverables Paid/Unpaid Losses	[1]	365,399
Reinsurance Recoverables Total		463,402
Collateral Held		$ 89,198

[1] Net of reserve for uncollectible reinsurance of approximately $12.1 million.