Reinsurance - Schedule of Unsecured Reinsurance Recoverables from Reinsurers (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Amounts recoverable from reinsurers		$ 90,133	$ 83,666
Funds withheld and letters of credit related to the above recoverables		(47,719)
Total unsecured amounts recoverable from reinsurers		42,414
Reinsurance Recoverable [Member] | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Amounts recoverable from reinsurers		90,133
Reinsurance Recoverable [Member] | Reinsurer Concentration Risk | Hannover Reinsurance Limited (Ireland) [Member] | A.M. Best Rating, A+ [Member]
Ceded Credit Risk [Line Items]
Amounts recoverable from reinsurers	[1]	36,905
Reinsurance Recoverable [Member] | Reinsurer Concentration Risk | Odyssey America Reinsurance Corporation [Member] | A.M. Best Rating, A [Member]
Ceded Credit Risk [Line Items]
Amounts recoverable from reinsurers		12,464
Reinsurance Recoverable [Member] | Reinsurer Concentration Risk | Allianz Risk Transfer AG (Bermuda) [Member] | A.M. Best Rating, A+ [Member]
Ceded Credit Risk [Line Items]
Amounts recoverable from reinsurers	[1]	7,551
Reinsurance Recoverable [Member] | Reinsurer Concentration Risk | Minnesota Workers’ Compensation Reinsurance Association [Member]
Ceded Credit Risk [Line Items]
Amounts recoverable from reinsurers	[1]	7,761
Reinsurance Recoverable [Member] | Reinsurer Concentration Risk | Clearwater Insurance [Member]
Ceded Credit Risk [Line Items]
Amounts recoverable from reinsurers	[2]	5,755
Reinsurance Recoverable [Member] | Reinsurer Concentration Risk | Other Reinsurers [Member]
Ceded Credit Risk [Line Items]
Amounts recoverable from reinsurers		$ 19,697

[1] Current participant in our 2018 reinsurance program.

[2] Subsidiary of Fairfax Financial is no longer rated by A.M. Best.