Off-Balance Sheet Credit Risk (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
General settlement period of securities transactions	2 days
Fair value of securities accepted as collateral permitted to sell or repledge	$ 2,600.0	$ 2,400.0
Fair value of collateral securities sold or repledged	346.2	233.7
Outstanding commitments to originate loans	214.8	160.2
Letters of credit outstanding	58.9	82.5
Unused Lines Of Credit [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Unused lines of credit to commercial and consumer borrowers	$ 745.1	$ 590.5
Standby Letters of Credit [Member] \| Maximum [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Letters of credit, expiration period	1 year

General settlement period of securities transactions.

No definition available.

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- Definition

Outstanding commitments to originate loans.

+ References

No definition available.

+ Details

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- Definition

Unused lines of credit to commercial and consumer borrowers.

+ References

No definition available.

+ Details

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- Definition

Fair value as of the balance sheet date of customer securities accepted as collateral by the entity that it is permitted by contract or custom to sell or re-pledge to counterparties as collateral.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

+ Details

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- Definition

Fair value as of the balance sheet date of customer securities accepted as collateral by the entity that it has sold or re-pledged to counterparties as collateral.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

+ Details

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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.

+ Details

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- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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- Definition

Period remaining on line of credit facility before it terminates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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