Lists of 10 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 231,174	$ 213,377
Reinsurance Recoverables Paid/Unpaid Losses	[1]	908,559
Reinsurance Recoverables Total		1,139,733
Collateral Held		171,616
Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		28,539
Reinsurance Recoverables Paid/Unpaid Losses	[1]	87,285
Reinsurance Recoverables Total		115,824
Collateral Held		$ 11,731
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		$ 23,116
Reinsurance Recoverables Paid/Unpaid Losses	[1]	73,548
Reinsurance Recoverables Total		96,664
Collateral Held		$ 7,088
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-
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 6,363
Reinsurance Recoverables Paid/Unpaid Losses	[1]	68,539
Reinsurance Recoverables Total		74,902
Collateral Held		$ 6,482
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+
Transatlantic Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 8,522
Reinsurance Recoverables Paid/Unpaid Losses	[1]	52,237
Reinsurance Recoverables Total		60,759
Collateral Held		$ 2,543
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		$ 11,094
Reinsurance Recoverables Paid/Unpaid Losses	[1]	46,158
Reinsurance Recoverables Total		57,252
Collateral Held		$ 4,898
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-
Aspen Insurance UK Ltd. | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 10,050
Reinsurance Recoverables Paid/Unpaid Losses	[1]	29,700
Reinsurance Recoverables Total		39,750
Collateral Held		$ 8,057
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
Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 10,623
Reinsurance Recoverables Paid/Unpaid Losses	[1]	27,670
Reinsurance Recoverables Total		38,293
Collateral Held		$ 11,100
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
Allied World Reinsurance | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 6,531
Reinsurance Recoverables Paid/Unpaid Losses	[1]	31,684
Reinsurance Recoverables Total		38,215
Collateral Held		$ 1,937
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-
Partner Reinsurance Europe | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 5,699
Reinsurance Recoverables Paid/Unpaid Losses	[1]	28,532
Reinsurance Recoverables Total		34,231
Collateral Held		$ 11,095
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+
Ace Property and Casualty Insurance Company | AM Best, A++ Rating | Standard & Poor's, AA Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 6,720
Reinsurance Recoverables Paid/Unpaid Losses	[1]	23,288
Reinsurance Recoverables Total		30,008
Collateral Held		$ 1,556
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
Top 10 Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 117,257
Reinsurance Recoverables Paid/Unpaid Losses	[1]	468,641
Reinsurance Recoverables Total		585,898
Collateral Held		66,487
Others
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		113,917
Reinsurance Recoverables Paid/Unpaid Losses	[1]	439,918
Reinsurance Recoverables Total		553,835
Collateral Held		$ 105,129

[1] Net of reserve for uncollectable reinsurance of approximately $12.4 million.