Regulatory Requirements - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Regulatory Requirements [Line Items]
Reserve balance with Federal Reserve Bank maintained by affiliate bank	$ 303.8	$ 297.4
Total Capital, Actual Ratio	14.04%
Tier 1 Capital For Capital Adequacy Purposes, Ratio	6.00%
Total Capital For Capital Adequacy Purposes, Ratio	8.00%
Tier 1 Capital, Actual Ratio	12.95%
Tier 1 Leverage for Capital Adequacy Purposes, Ratio	4.00%
Tier 1 Leverage, Actual Ratio	9.94%
Tier 1 Capital To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%
Tier 1 Capital To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%
Tier 1 Leverage To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%
Common Equity Tier One Capital [Member]
Regulatory Requirements [Line Items]
Common equity capital conservation buffer to risk weighted asset	2.50%
Tier 1 Capital For Capital Adequacy Purposes, Ratio	4.50%
Tier 1 Capital, Actual Ratio	12.95%
Tier 1 Capital To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%

Affiliate banks average reserve balances with the federal reserve bank.

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Regulatory requirements.

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Tier one capital conservation buffer to risk weighted assets required under regulatory.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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-Publisher FASB
-Name Accounting Standards Codification
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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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