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Benefit Plans Narrative (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) yr	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Pension Plans [Abstract]
Number of compensation years included in benefit calculation | yr	3
Expected future working lifetime of participants	12 years 7 months 13 days
Market gains (losses)	$ 898,000	$ 3,000
Return on investments for past 10 years, percent	5.41%
Investments benchmark for past 10 years, percent	5.47%
Return on investments for past five years, percent	8.37%
Investments benchmark for past five years, percent	8.48%
Flexible Spending Plan [Abstract]
Annual maximum contribution limit under Health Care Spending Account plan	$ 2,500.0
Annual maximum contribution limit under Dependent Care Spending Account plan	5,000
Deferral Plan [Abstract]
Employer contributions to deferral plan	$ 1,242,000	1,149,000	$ 1,101,000
Fixed Income Funds [Member]
Pension Plans [Abstract]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum	25.00%
Bonds [Member]
Pension Plans [Abstract]
Actual plan asset allocations	34.00%
Cash and Cash Equivalents [Member]
Pension Plans [Abstract]
Actual plan asset allocations	8.00%
Equity Securities [Member]
Pension Plans [Abstract]
Actual plan asset allocations	58.00%
Private Corporation Assets [Member]
Pension Plans [Abstract]
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum	5.00%
Minimum [Member]
Pension Plans [Abstract]
Period for performance standards	3 years
Maximum [Member]
Pension Plans [Abstract]
Period for performance standards	5 years
Executive Supplemental Retirement Plan [Member]
Pension Plans [Abstract]
Projected benefit obligation	$ 19,051,000	16,566,000
Net periodic pension cost	1,729,000	1,542,000	1,300,000
Executives and Non-employee Directors [Member] | Deferred Compensation Arrangement with Individual, by Type of Compensation, Pension and Other Postretirement Benefits [Member]
Special Deferral Election Plan and Deferral Election Program [Abstract]
Deferrals by executives and non-employee directors	$ 4,250,000	$ 4,073,000	$ 3,463,000

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Deferred Compensation Arrangement With Individual, Deferral Amount

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Health Care Spending Account, Annual Maximum Employee Contribution

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Pension Plans [Abstract]

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Special Deferral Election Plan and Deferral Election Program [Abstract]

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The amount of payment into the fund established for purposes of making future disbursements to the individual in accordance with the terms and understanding of the deferred compensation arrangement.

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The amount of gains or losses recognized in net periodic benefit cost.

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-Name Accounting Standards Codification
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1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.

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The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

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Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis.

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Target allocation minimum percentage of investments of this type to total plan assets presented on a weighted average basis.

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Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

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