Derivative Instruments and Hedging Activities - Additional Information (Detail) $ in Millions	9 Months Ended
Sep. 30, 2017 USD ($) Derivative Swap	Dec. 31, 2016 USD ($) Derivative Swap
Derivative Instruments, Gain (Loss) [Line Items]
Reclassification adjustment from accumulated other comprehensive income	$ 1.2
Assets needed for immediate settlement of derivatives in a net liability position	7.3
Minimum collateral with counterparties	$ 8.3
Minimum
Derivative Instruments, Gain (Loss) [Line Items]
Derivative expiration period	30 days
Maximum
Derivative Instruments, Gain (Loss) [Line Items]
Derivative expiration period	60 days
Not Designated as Hedging Instrument \| Residential mortgage loan commitments
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 5.4	$ 3.6
Interest Rate Swaps \| Derivative Designated as Cash Flow Hedges \| Cash Flow Hedges
Derivative Instruments, Gain (Loss) [Line Items]
Number of outstanding interest rate derivatives (derivative) \| Derivative	9	4
Aggregate notional amount	$ 550.0	$ 325.0
Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 793.6	$ 716.6
Number of interest rate swaps (swap) \| Swap	76	76
Credit Contract \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 53.3	$ 44.9

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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The aggregate fair value of assets needed to immediately settle the derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959

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The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959

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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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The estimated value of gains (losses), net anticipated to be transferred in the future from accumulated other comprehensive income into earnings.

+ References

No definition available.

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- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959

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Period remaining until the derivative contract matures, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Number of interest rate derivative instruments held by the entity at the reporting date.

+ References

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