Lists of 10 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 228,569	$ 213,377
Reinsurance Recoverables Paid/Unpaid Losses	[1]	882,259
Reinsurance Recoverables Total		1,110,828
Collateral Held		180,664
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		5,770
Reinsurance Recoverables Paid/Unpaid Losses	[1]	61,413
Reinsurance Recoverables Total		67,183
Collateral Held		$ 5,423
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		$ 29,333
Reinsurance Recoverables Paid/Unpaid Losses	[1]	90,005
Reinsurance Recoverables Total		119,338
Collateral Held		$ 9,564
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		$ 18,987
Reinsurance Recoverables Paid/Unpaid Losses	[1]	71,062
Reinsurance Recoverables Total		90,049
Collateral Held		$ 8,069
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		$ 8,587
Reinsurance Recoverables Paid/Unpaid Losses	[1]	50,487
Reinsurance Recoverables Total		59,074
Collateral Held		$ 2,588
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		$ 10,991
Reinsurance Recoverables Paid/Unpaid Losses	[1]	46,751
Reinsurance Recoverables Total		57,742
Collateral Held		$ 5,171
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		$ 6,511
Reinsurance Recoverables Paid/Unpaid Losses	[1]	32,422
Reinsurance Recoverables Total		38,933
Collateral Held		$ 1,637
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		$ 12,096
Reinsurance Recoverables Paid/Unpaid Losses	[1]	22,509
Reinsurance Recoverables Total		34,605
Collateral Held		$ 11,172
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		$ 8,921
Reinsurance Recoverables Paid/Unpaid Losses	[1]	29,093
Reinsurance Recoverables Total		38,014
Collateral Held		$ 8,775
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
Scor Global P&C SE | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 5,938
Reinsurance Recoverables Paid/Unpaid Losses	[1]	25,228
Reinsurance Recoverables Total		31,166
Collateral Held		$ 6,393
Scor Global P&C SE | AM Best, A+ Rating | AM Best | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		A+
Scor Global P&C SE | 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		$ 5,257
Reinsurance Recoverables Paid/Unpaid Losses	[1]	25,472
Reinsurance Recoverables Total		30,729
Collateral Held		$ 11,408
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		$ 112,391
Reinsurance Recoverables Paid/Unpaid Losses	[1]	454,442
Reinsurance Recoverables Total		566,833
Collateral Held		70,200
All Others
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		116,178
Reinsurance Recoverables Paid/Unpaid Losses	[1]	427,817
Reinsurance Recoverables Total		543,995
Collateral Held		$ 110,464

[1] Net of reserve for uncollectible reinsurance of approximately $12.6 million.