Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Our Reinsurers (Detail) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses		$ 1,703,900		$ 1,682,000
Allowance for credit losses		(6,900)	$ (6,900)	0
Reinsurance recoverables		1,696,996		$ 1,681,962
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[1],[2]	$ 1,703,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[1],[2]	100.00%
Allowance for credit losses	[2]	$ (6,900)
Reinsurance recoverables	[2]	1,697,000
Syndicates at Lloyd's of London | AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 143,700
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	8.40%
Kane SAC Ltd, Rondout Segregated Account | Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3],[4]	$ 113,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	6.60%
PartnerRe Ltd | AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 110,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.50%
Fairfax Financial Holdings Ltd | AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 102,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.00%
Swiss Reinsurance Company | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 99,800
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	5.90%
Third Point Reinsurance Group | AM Best, A- Rating | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 79,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	4.70%
RenaissanceRe Holdings Ltd | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 102,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.00%
W.R. Berkley Corporation | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 73,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	4.30%
Chubb | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 66,600
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.90%
Liberty Mutual Insurance Company | AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 60,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.50%
All other reinsurers | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2]	$ 751,800
Reinsurance recoverable as percentage of total reinsurance recoverables	[2]	44.20%
Ceded Credit Risk, Secured | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	34.90%
Reinsurance recoverables	[2],[4]	$ 595,000

[1] Approximately 72 percent of our reinsurance recoverables balance as of September 30, 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 the A.M. Best Company, Inc. financial strength rating for the applicable reinsurance subsidiary or subsidiaries from which the reinsurance recoverable is due.

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