Information Regarding Concentration of Reinsurance Recoverables and Ratings Profile of its Reinsurers (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Reinsurance Recoverable		$ 1,272,219	$ 1,249,948
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[1],[2]	$ 1,272,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[1],[2]	100.00%
Reinsurance Recoverable | Reinsurer Concentration Risk | Ceded Credit Risk, Secured
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[3]	$ 183,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	14.40%
Reinsurance Recoverable | Reinsurer Concentration Risk | All other reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2]	$ 392,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2]	30.70%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Swiss Reinsurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 172,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	13.50%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Renaissance Re Holdings Limited
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 91,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	7.20%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | W.R. Berkley Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 89,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	7.10%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Allianz SE
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 63,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.00%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Hannover Ruck SE
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 46,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	3.70%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | PartnerRe Limited
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 116,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	9.10%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | Syndicates at Lloyd's of London
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 105,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	8.30%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | Allied World Assurance Company Holdings Ag
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4],[5]	$ 60,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4],[5]	4.80%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | Fairfax Financial Holdings Limited
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4],[5]	$ 43,600
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4],[5]	3.40%
A++ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Chubb Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 91,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	7.20%

[1] Approximately 94 percent of our reinsurance recoverables balance as of December 31, 2016 was due from reinsurers having an A.M. Best financial strength rating of A (Excellent) or higher.

[2] Reinsurance recoverables reflect amounts due from one or more reinsurance subsidiaries of the listed company.

[3] Represents reinsurance recoverables secured by funds held, trust agreements and letters of credit.

[4] Represents the A.M. Best financial strength rating for the applicable reinsurance subsidiary or subsidiaries from which the reinsurance recoverable is due.

[5] In December 2016, Fairfax Financial Holdings Ltd announced its agreement to acquire Allied World Assurance Company Holdings, AG.