Historical Loss and LAE Development - Additional Information (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
(Favorable)/Unfavorable Reserve Development
Unpaid losses and LAE, Ceded	$ 602,154	$ 399,960	$ 443,729	$ 384,517
Favorable development on prior years' loss reserves	$ 96,968	$ 70,048
Accounting Standards Update 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
(Favorable)/Unfavorable Reserve Development
Unpaid losses and LAE, Ceded				$ 1,345

Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

Name:	us-gaap_ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant