Employee Benefit Plan - Narrative (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) pension_plan shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)
Defined Contribution Plan Disclosure [Line Items]
Number of non-contributory defined-contribution retirement plans \| pension_plan	2
Common Stock, issued (in shares) \| shares	15,301,524	15,171,816
Common stock, value	$ 1,530	$ 1,517
Expenses related to defined-contribution plans	$ 4,100	$ 3,800	$ 2,400
Expected long-term return on plan assets (percent)	7.25%	7.25%	7.25%
Employee stock ownership plan - expense recognized	$ 5,800	$ 4,900	$ 4,400
Incentive stock options (ISOPs), maximum contribution, percent	4.00%
Matching contributions to 401(k) plan cost	$ 2,500	2,400	$ 2,300
Corporate owned life insurance	$ 80,106	$ 77,905
DB Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Common Stock, issued (in shares) \| shares	42,192	42,192
Common stock, value	$ 3,400	$ 4,000
Accumulated benefit obligation	82,700	77,300
Unfunded status recognized in other liabilities	2,594	5,497
Rate of compensation increase (percent)			5.00%
Increase in service and interest costs	1,500
Increase in benefit obligation amount	48
Decrease in service and interest costs	1,300
Decrease in benefit obligation, amount	$ 43
Expected long-term return on plan assets (percent)	7.25%
DB Pension Plan \| Maximum
Defined Contribution Plan Disclosure [Line Items]
Rate of compensation increase (percent)	6.10%
DB Pension Plan \| Minimum
Defined Contribution Plan Disclosure [Line Items]
Rate of compensation increase (percent)	4.50%
DC Retirement Plan
Defined Contribution Plan Disclosure [Line Items]
Requisite service period	1 year
Maximum annual contribution, percent of account balances, percent	4.00%
Accumulated benefit obligation	$ 9,000	9,100
Unfunded status recognized in other liabilities	$ 8,995	$ 9,121
Rate of compensation increase (percent)	5.00%	5.00%	5.00%
SERPs
Defined Contribution Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 26,100	$ 23,400
Unfunded status recognized in other liabilities	$ 26,142	$ 23,399
Rate of compensation increase (percent)	5.00%	5.00%	5.00%
Deferred Compensation Arrangement with Individual, by Type of Compensation, Pension and Other Postretirement Benefits
Defined Contribution Plan Disclosure [Line Items]
Benefit liability	$ 1,500	$ 1,400
Compensation expense	$ 115	$ 110

Deferred Compensation Arrangement with Individual, Requisite Service Period To Participate In Pension Plan

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The amount of matching contributions for the 401(k) plan the company has made during the period.

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Defined Contribution Plan, Number Of Non-contributory Defined-contribution Retirement Plans

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Employee Stock Ownership Plan (ESOP), Incentive Stock Options (ISOPs), Maximum Employer Matching Contribution, Percent of Match

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The carrying amount of a life insurance policy on an officer, executive or employee for which the reporting entity (a bank) is entitled to proceeds from the policy upon death of the insured or surrender of the insurance policy.

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Amount of liability, recognized in statement of financial position, for defined benefit pension plan, classified as noncurrent. Excludes other postretirement benefit plan.

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Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

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Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

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Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate.

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Amount of decrease in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point decrease in assumed health care cost trend rate.

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Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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Amount of cost for defined contribution plan.

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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.

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Amount of cost (reversal of cost) for employee benefits and share-based payment arrangement. Includes, but is not limited to, pension, other postretirement, postemployment and termination benefits.

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The amount of plan compensation cost recognized during the period.

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