Regulatory Requirements - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Regulatory Requirements [Line Items]
Reserve balance with Federal Reserve Bank maintained by affiliate bank	$ 1,200.0	$ 560.9
Tier 1 Capital For Capital Adequacy Purposes, Ratio	6.0
Total Capital For Capital Adequacy Purposes, Ratio	8.0
Tier 1 Capital, Actual Ratio	12.10
Total Capital, Actual Ratio	14.26
Tier 1 Leverage for Capital Adequacy Purposes, Ratio	4.0
Tier 1 Leverage, Actual Ratio	8.37
Tier 1 Capital To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.0
Tier 1 Capital To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.0
Tier 1 Leverage To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.0
Common Equity Tier One Capital [Member]
Regulatory Requirements [Line Items]
Tier 1 Capital For Capital Adequacy Purposes, Ratio	4.5
Tier 1 Capital, Actual Ratio	12.10
Tier 1 Capital To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.5

Affiliate banks average reserve balances with the federal reserve bank.

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Regulatory requirements.

+ References

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- Definition

Ratio of minimum total risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958570-112826

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826

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Ratio of minimum total risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826

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Ratio of total risk-based capital to risk-weighted assets as defined by regulatory framework.

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Ratio of minimum Tier 1 leverage capital to average assets required for capital adequacy as defined by regulatory framework.

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