Goodwill and Other Intangible Assets - Goodwill and Other Intangibles (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Total	$ 100,428	$ 111,110
Core Deposit Premium
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	144,202	146,355
Accumulated amortization	(56,491)	(48,992)
Total	87,711	97,363	$ 111,808
Books of Business Intangible
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	19,937	19,937
Accumulated amortization	(7,220)	(6,190)
Total	$ 12,717	$ 13,747	$ 15,532

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
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-Section 50
-Paragraph 2
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Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 928
-SubTopic 340
-Section 50
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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 926
-SubTopic 20
-Section 50
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