Goodwill and Other Intangibles - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Goodwill And Other Intangible Assets [Line Items]
Goodwill	$ 174,518	$ 180,867	$ 180,867
Sale of component of business segment	(6,349)
Commercial Banking [Member]
Goodwill And Other Intangible Assets [Line Items]
Goodwill	59,419	59,419	59,419
Institutional Banking [Member]
Goodwill And Other Intangible Assets [Line Items]
Goodwill	51,332	51,332	51,332
Personal Banking [Member]
Goodwill And Other Intangible Assets [Line Items]
Goodwill	63,767	$ 70,116	$ 70,116
Sale of component of business segment	$ (6,349)

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10)(1))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

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