Retirement Plans - AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Pension Plans
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax [Roll Forward]
Net unrecognized actuarial (loss) gain at beginning of period	$ (280,948)	$ (268,515)
Amortization of net loss (gain)	10,836	10,914
Net loss arising during the year	2,311	(24,709)
Currency translation	(626)	1,362
Net unrecognized actuarial (loss) gain at end of period	(268,427)	(280,948)
Post-Retirement Medical Benefits Plan
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax [Roll Forward]
Net unrecognized actuarial (loss) gain at beginning of period	304	456
Amortization of net loss (gain)	(152)	(152)
Net loss arising during the year	0	0
Currency translation	0	0
Net unrecognized actuarial (loss) gain at end of period	$ 152	$ 304