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