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Benefit Plans Narrative (Details) | 12 Months Ended
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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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- Definition Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Number of Compensation Years + References No definition available. + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Projected Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6522206 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Postretirement Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503904 + Details
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- Definition Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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