Contingencies - Additional Information (Details) $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($) Lawsuit	Sep. 30, 2017 USD ($)
Loss Contingencies [Line Items]
Unused commitments to extend credit	$ 3,666.0	$ 3,143.0
Pending or Threatened Litigation, Stanford Financial Group [Member]
Legal Proceedings [Abstract]
Lawsuits naming entity as defendant, number \| Lawsuit	3
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Potential exposure to credit loss in the event of nonperformance	$ 102.9	105.6
Letters of credit, maturity term - maximum	3 years
Collateral held, fair value	$ 31.2	$ 30.2

Legal Proceedings [Abstract]

No definition available.

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- Definition

The maximum maturity (in years) of outstanding letters of credit issued by company.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-Publisher FASB
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-Topic 460
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

Number of pending claims pertaining to a loss contingency.

+ References

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- Definition

The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

Unused amount of commitments to extend credit to customers.

+ References

No definition available.

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