Pension and Post-retirement Plans - Changes in Benefit Obligation and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension obligations
Change in plans' benefit obligations
Pension plan's benefit obligation - beginning of year	$ 110,410	$ 64,435
Sonneborn acquisition	0	31,686
Service cost	3,929	4,135	$ 4,420
Interest cost	2,772	3,026	2,249
Actuarial loss	8,391	5,161
Benefits paid	(1,558)	(1,132)
Curtailment	(4,078)	0
Contractual termination benefits	915	0
Transfer from other plans	479	330
Foreign currency exchange rate changes	5,360	2,769
Pension plan's benefit obligation - end of year	126,620	110,410	64,435
Change in pension plans assets
Fair value of plan assets - beginning of year	105,358	62,462
Acquisition of Sonneborn	0	29,376
Return on plans assets	10,936	7,947
Employer contributions	3,487	3,681
Transfer payments	479	330
Foreign currency exchange rate changes	5,248	2,694
Fair value of plan assets - end of year	123,950	105,358	62,462
Under-funded balance	(2,670)	(5,052)
Accrued post-retirement plan liability	(2,670)	(5,052)
Cumulative actuarial gain (loss)	(1,658)	(3,155)
Post-retirement healthcare obligations
Change in plans' benefit obligations
Pension plan's benefit obligation - beginning of year	31,273	26,880
Sonneborn acquisition	0	877
Service cost	1,616	1,582	1,648
Interest cost	870	1,029	938
Actuarial loss	1,131	2,412
Benefits paid	(1,766)	(2,028)
Foreign currency exchange rate changes	354	521
Pension plan's benefit obligation - end of year	33,478	31,273	26,880
Change in pension plans assets
Fair value of plan assets - beginning of year	0	0
Employer contributions	1,742	2,003
Participant contributions	24	25
Fair value of plan assets - end of year	0	0	$ 0
Under-funded balance	(33,478)	(31,273)
Accrued liabilities	(1,946)	(1,817)
Other long-term liabilities	(31,532)	(29,456)
Accrued post-retirement plan liability	(33,478)	(31,273)
Cumulative actuarial gain (loss)	(1,523)	(197)
Prior service credit	(18,511)	(21,992)
Total	$ 16,988	$ 21,795