Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Our Reinsurers (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses		$ 1,835,200	$ 1,789,000
Allowance for credit losses		(6,000)	(7,900)	$ (6,800)	$ 0
Reinsurance recoverables		1,829,187	$ 1,781,096
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[1],[2]	$ 1,835,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[1],[2]	100.00%
Allowance for credit losses	[2]	$ (6,000)
Reinsurance recoverables	[2]	1,829,200
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]	$ 152,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	8.30%
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]	$ 129,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	7.10%
PartnerRe Limited | A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 119,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.50%
RenaissanceRe Holdings Ltd | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 119,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.50%
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]	$ 108,700
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]	$ 97,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	5.30%
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]	$ 79,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	4.30%
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]	4.00%
Chubb | 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]	3.90%
Kane SAC Ltd, Bowery Segregated Account | Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3],[4]	$ 61,600
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	3.40%
All other reinsurers | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2]	$ 822,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2]	44.80%
Ceded Credit Risk, Secured | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	38.00%
Reinsurance recoverables	[2],[4]	$ 697,800

[1] Approximately 71 percent of our reinsurance recoverables balance as of March 31, 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.