Pension Plan and Postretirement Benefits Other Than Pension - Changes in net funded status, disclosure of amounts recognized in the balance sheet, and the assumptions used to determine the benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan and Postretirement Benefits Other than Pension
Curtailment gain			$ 31,600
Amounts recognized in the statement of financial position:
Noncurrent liabilities	$ (3,145)	$ (798)
Pension Benefits
Pension Plan and Postretirement Benefits Other than Pension
Final number of years of employee's compensation to determine the benefits payable	10 years
Reduction of the accrued pension liability			(30,000)
Curtailment gain			31,621
Change in projected benefit obligation:
Net benefit obligation at beginning of year	$ 154,528	184,245	180,921
Service cost			8,367
Interest cost	6,146	5,986	6,248
Benefits paid	(13,221)	(13,690)	(8,511)
Actuarial loss (gain)	39,027	(22,013)	28,841
Curtailment gain			(31,621)
Net benefit obligation at end of year	186,480	154,528	184,245
Change in plan assets:
Fair value of plan assets at beginning of year	162,999	170,881	144,529
Actual return on plan assets	34,125	1,808	24,863
Employer contributions		4,000	10,000
Benefits paid	(13,221)	(13,690)	(8,511)
Fair value of plan assets at end of year	183,903	162,999	170,881
Funded status at end of year	(2,577)	8,471	(13,364)
Amounts recognized in the statement of financial position:
Noncurrent assets		8,471
Noncurrent liabilities	(2,577)		(13,364)
Net amount recognized at end of year	$ (2,577)	$ 8,471	$ (13,364)
Weighted average assumptions used to determine benefit obligation at December 31:
Discount rate (as a percent)	3.32%	4.45%	3.76%
Other Postretirement Benefits
Pension Plan and Postretirement Benefits Other than Pension
Age of employees after which the plan does not provide benefits	65 years
Change in projected benefit obligation:
Net benefit obligation at beginning of year	$ 1,048	$ 2,195	$ 2,446
Interest cost	33	54	58
Benefits paid	(677)	(602)	(954)
Actuarial loss (gain)	47	(965)	139
Retiree contributions	275	366	506
Net benefit obligation at end of year	726	1,048	2,195
Change in plan assets:
Employer contributions	402	236	448
Retiree contributions	275	366	506
Benefits paid	(677)	(602)	(954)
Funded status at end of year	(726)	(1,048)	(2,195)
Amounts recognized in the statement of financial position:
Current liabilities	(158)	(250)	(422)
Noncurrent liabilities	(568)	(798)	(1,773)
Net amount recognized at end of year	$ (726)	$ (1,048)	$ (2,195)
Weighted average assumptions used to determine benefit obligation at December 31:
Discount rate (as a percent)	2.87%	4.08%	3.28%