Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Our Reinsurers (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Dec. 31, 2019
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses		$ 1,635,400	$ 1,682,000
Allowance for credit losses		(6,800)
Reinsurance recoverables		1,628,649	$ 1,681,962
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[1],[2]	$ 1,635,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[1],[2]	100.00%
Allowance for credit losses	[2]	$ (6,800)
Reinsurance recoverables	[2]	$ 1,628,600
Reinsurance Recoverable | Reinsurer Concentration Risk | Ceded Credit Risk, Secured
Ceded Credit Risk [Line Items]
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	40.30%
Reinsurance recoverables	[2],[3]	$ 659,600
Reinsurance Recoverable | Reinsurer Concentration Risk | All other reinsurers
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2]	$ 741,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2]	45.50%
AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | Syndicates at Lloyd's of London
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 132,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	8.10%
AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | PartnerRe Limited
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 97,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	6.00%
AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | Fairfax Financial Holdings Limited
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 90,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.50%
AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | Liberty Mutual
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 55,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	3.40%
Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk | Kane SAC Ltd, Rondout Segregated Account
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3],[4]	$ 121,800
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	7.40%
Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk | Kane SAC Ltd, Bowery Segregated Account
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3],[4]	$ 77,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	4.70%
AM Best, A+ Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | Swiss Reinsurance Company
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 88,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.40%
AM Best, A+ Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | RenaissanceRe Holdings Ltd
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 75,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	4.60%
AM Best, A+ Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | W.R. Berkley Corporation
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 67,700
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	4.10%
AM Best, A- Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | Third Point Reinsurance Group
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 87,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.30%

[1] Approximately 66 percent of our reinsurance recoverables balance as of March 31, 2020 was due from reinsurers having an A.M. Best Company, Inc. financial strength rating of A (Excellent) or higher, with a majority of the other reinsurance recoverables being secured by funds held, trust agreements or letters of credit.

[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 or letters of credit.

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