Pension and Other Postretirement Benefits - Recognized on consolidated balance sheets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets and liabilities recognized on consolidated balance sheets:
Accrued pension and other postretirement benefits	$ (118,852)	$ (102,077)
Pension Benefits
Assets and liabilities recognized on consolidated balance sheets:
Other noncurrent assets	13,866	15,071
Accrued compensation and benefits	(5,357)	(10,391)
Accrued pension and other postretirement benefits	(102,834)	(88,386)
Net liability recognized	(94,325)	(83,706)
Accumulated other comprehensive income (loss) on consolidated balance sheets that have not yet been recognized as components of net periodic benefit cost:
Net actuarial gain (loss) at beginning of year	(39,125)	(44,892)	$ (33,736)
Actuarial gain (loss)	(27,123)	(882)	(14,554)
Prior service cost	(1,464)	(285)	0
Curtailment impact	0	(97)	0
Settlement impact	4,324	4,982	2,740
Amortization of actuarial (gain) loss	1,530	1,687	804
Amortization of prior service cost (credit)	247	175	175
Foreign currency translation	(190)	187	(321)
Net actuarial gain (loss) at end of year	(61,801)	(39,125)	(44,892)
Expected prior service cost (credit) and actuarial (gain) loss will be amortized in next fiscal year
Expected prior service cost (credit) will be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost in the next fiscal year	300
Expected actuarial (gain) loss will be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost in the next fiscal year	(3,800)
Postretirement Benefits
Assets and liabilities recognized on consolidated balance sheets:
Other noncurrent assets	0	0
Accrued compensation and benefits	(807)	(721)
Accrued pension and other postretirement benefits	(16,018)	(13,691)
Net liability recognized	(16,825)	(14,412)
Accumulated other comprehensive income (loss) on consolidated balance sheets that have not yet been recognized as components of net periodic benefit cost:
Net actuarial gain (loss) at beginning of year	12,315	12,392	8,055
Actuarial gain (loss)	(2,288)	1,600	5,075
Prior service cost	0	0	0
Curtailment impact	0	0	0
Settlement impact	0	0	0
Amortization of actuarial (gain) loss	(1,436)	(1,540)	(601)
Amortization of prior service cost (credit)	(137)	(137)	(137)
Foreign currency translation	0	0	0
Net actuarial gain (loss) at end of year	8,454	$ 12,315	$ 12,392
Expected prior service cost (credit) and actuarial (gain) loss will be amortized in next fiscal year
Expected prior service cost (credit) will be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost in the next fiscal year	(100)
Expected actuarial (gain) loss will be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost in the next fiscal year	$ 900