Pension and Post-retirement Plans - Net Periodic Credit (Details) - Post-retirement healthcare obligations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,616	$ 1,582	$ 1,648
Interest cost	870	1,029	938
Amortization of prior service credit	(3,481)	(3,481)	(3,481)
Amortization of gain	(83)	(106)	0
Net periodic post-retirement credit	$ (1,078)	$ (976)	$ (895)