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Pension Benefit and Retirement Health and Life Insurance Benefits (Additional Information) (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Projected benefit obligation of plan with accumulated benefit obligation in excess of plan assets |  |  | $ 155,800 | $ 148,600
Accumulated benefit obligation of plan with accumulated benefit obligation in excess of plan assets |  |  | 155,800 | 148,600
Fair value of the plan assets of plan with accumulated benefit obligation in excess of plan assets |  |  | 147,100 | 140,100
Projected benefit obligation of plan with plan assets in excess of accumulated benefit obligation |  |  | 30,000 | 29,100
Accumulated benefit obligation of plan with plan assets in excess of accumulated benefit obligation |  |  | 30,000 | 29,100
Fair value of the plan assets of plan with plan assets in excess of accumulated benefit obligation |  |  | $ 33,000 | $ 31,700
Change in discount rate |  |  |  | 4.00%
Health care cost trend rate annual change |  |  | (0.25%) |
Percentage point change in assumed health care cost trend rates |  |  | 1.00% |
Health care cost, employees age |  |  | 65 years |
Retirees of 65 Years Old or Younger |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Health care cost trend rate assumed for next fiscal year |  |  | 7.25% | 7.50%
Ultimate health care cost trend rate |  |  | 4.50% |
Retirees of 65 Years Old or Older |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Health care cost trend rate assumed for next fiscal year |  |  | 7.25% | 7.50%
Ultimate health care cost trend rate |  |  | 4.50% |
Pension Benefits |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net loss to be recognized over next twelve months |  |  | $ 1,800 |
Historical return on plan assets over 20 years |  |  | 7.40% |
Expected long-term rate of return on plan assets |  |  | 4.90% |
Employer contributions |  |  | $ 372 | $ 344
Pension Benefits | Equity Securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Plan asset allocations |  |  | 4.00% | 27.00%
Pension Benefits | Debt Securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Plan asset allocations |  |  | 96.00% | 73.00%
Retirement Health and Life Insurance Benefits |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net loss to be recognized over next twelve months |  |  | $ 1,600 |
Employer contributions |  |  | $ 533 | $ 498
Bear Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined Benefit Pension Plan, Required Employer Contribution | $ 400 |  |  |
Employer contributions |  | $ 300 |  |

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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Change in Discount Rate + References No definition available. + Details
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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Target Return Rate + References No definition available. + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + 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 (d)(5)(ii) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(01) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount included in accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan expected to be recognized in net periodic benefit (cost) credit for fiscal year following most recent annual statement of financial position. + 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 (s) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status. + 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 (l) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of accumulated benefit obligation for defined benefit pension plan with accumulated benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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- Definition Fair value of plan assets for defined benefit pension plan with accumulated benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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- Definition Amount of projected benefit obligation for defined benefit pension plan with accumulated benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=77993564&loc=d3e4179-114921 + Details
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