Purchased Loans - Additional Information (Details)	12 Months Ended
Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($) loan	Dec. 31, 2015 USD ($)
Loans and Allowance for Loan Losses (Textual) [Abstract]
Restructured loans discontinued past due period	90 days
Allowance for loan losses attributable to restructured loans	$ 46,211,000	$ 42,737,000
Purchased
Loans and Allowance for Loan Losses (Textual) [Abstract]
Nonperforming loans charged off past due period	90 days
Number of restructured loans \| loan	3	3
Restructured loans discontinued past due period	90 days	90 days
Average recorded investment in impaired loans	$ 7,687,000	$ 6,594,000	$ 2,860,000
Interest income recognized on impaired loans			125,000
Interest income recognized on credit-deteriorated loans	234,000	48,000
Modifications, recorded investment	8,965,000	4,028,000	$ 3,201,000
Modifications, subsequent default, recorded investment	212,000	54,000
Remaining availability under commitments to lend additional funds on restructured loans	9,000	3,000
Purchased \| Nonaccruing Loans
Loans and Allowance for Loan Losses (Textual) [Abstract]
Outstanding balance of restructured loans	128,000	56,000
Purchased \| Restructured Loans
Loans and Allowance for Loan Losses (Textual) [Abstract]
Outstanding balance of restructured loans	523,000	1,206,000
Allowance for loan losses attributable to restructured loans	$ 103,000	$ 35,000

Loans and the allowance for loan losses.

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Nonperforming loans charged off past due period.

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Number of Restructured Loans

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No definition available.

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Maximum duration past due for restructured loans to be considered performing.

+ References

No definition available.

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The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
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- Definition

Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
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Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

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Amount of interest income, recognized on cash basis, on financing receivables, for which there is no related allowance for credit losses and are recognized as impaired.

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- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Reflects the unfunded portion of contractual agreements to lend funds to borrowers for loans whose terms have been modified by troubled debt restructuring.

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The amount of interest income on impaired and nonperforming loans that is due to the change in present value attributable to the passage of time.

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-Publisher FASB
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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

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