Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Our Reinsurers (Detail) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses		$ 1,876,000		$ 1,789,000
Allowance for credit losses		(2,600)	$ (6,000)	(7,900)	$ (6,900)	$ (6,800)	$ 0
Reinsurance recoverables		1,873,447		$ 1,781,096
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[1],[2]	$ 1,876,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[1],[2]	100.00%
Allowance for credit losses	[2]	$ (2,600)
Reinsurance recoverables	[2]	1,873,400
Syndicates at Lloyd's of London | A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 148,800
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	7.90%
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]	$ 122,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	6.50%
PartnerRe Limited | A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 126,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.70%
RenaissanceRe Holdings Ltd | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 125,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.70%
Fairfax Financial Holdings Limited | A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 110,800
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	5.90%
Swiss Reinsurance Company | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 97,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	5.20%
Third Point Reinsurance Group | A- (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3],[4]	$ 71,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	3.80%
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]	$ 71,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.80%
Chubb | A++ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 72,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.80%
AXA XL | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 61,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.30%
All other reinsurers | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2]	$ 868,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2]	46.40%
Ceded Credit Risk, Secured | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	38.60%
Reinsurance recoverables	[2],[4]	$ 723,400

[1] Approximately 75 percent of our reinsurance recoverables balance as of June 30, 2021 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.