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, 2018	Dec. 31, 2017	Dec. 31, 2016
Amounts recognized in the statement of financial position:
Noncurrent liabilities	$ (798)	$ (15,137)
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	$ 184,245	180,921	$ 210,783
Service cost		8,367	12,199
Interest cost	5,986	6,248	9,432
Benefits paid	(13,690)	(8,511)	(52,288)
Actuarial (gain) loss	(22,013)	28,841	(19,886)
Curtailment gain		(31,621)
Settlement loss			20,681
Net benefit obligation at end of year	154,528	184,245	180,921
Change in plan assets:
Fair value of plan assets at beginning of year	170,881	144,529	173,885
Actual return on plan assets	1,808	24,863	2,932
Employer contributions	4,000	10,000	20,000
Benefits paid	(13,690)	(8,511)	(52,288)
Fair value of plan assets at end of year	162,999	170,881	144,529
Funded status at end of year	8,471	(13,364)	(36,392)
Amounts recognized in the statement of financial position:
Noncurrent assets	8,471
Noncurrent liabilities		(13,364)	(36,392)
Net amount recognized at end of year	$ 8,471	$ (13,364)	$ (36,392)
Weighted average assumptions used to determine benefit obligation at December 31:
Discount rate (as a percent)	4.45%	3.76%	4.39%
Rate of compensation increase (as a percent)			5.12%
Other Postretirement Benefits
Pension Plan and Postretirement Benefits Other than Pension
Age of employees after which the plan does not provide benefits	65 years
Curtailment gain			$ 8,475
Change in projected benefit obligation:
Net benefit obligation at beginning of year	$ 2,195	$ 2,446	8,421
Service cost			555
Interest cost	54	58	297
Benefits paid	(602)	(954)	(674)
Actuarial (gain) loss	(965)	139	1,790
Retiree contributions	366	506	532
Curtailment gain			(8,475)
Net benefit obligation at end of year	1,048	2,195	2,446
Change in plan assets:
Employer contributions	236	448	142
Retiree contributions	366	506	532
Benefits paid	(602)	(954)	(674)
Funded status at end of year	(1,048)	(2,195)	(2,446)
Amounts recognized in the statement of financial position:
Current liabilities	(250)	(422)	(458)
Noncurrent liabilities	(798)	(1,773)	(1,988)
Net amount recognized at end of year	$ (1,048)	$ (2,195)	$ (2,446)
Weighted average assumptions used to determine benefit obligation at December 31:
Discount rate (as a percent)	4.08%	3.28%	3.46%