Pension and Post-Retirement Benefits - Net Pension and Post-Retirement Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Pension Plans, Postretirement and Other Employee Benefits
Service cost	$ 1,304	$ 1,307	$ 1,037
Interest cost	5,375	4,632	3,971
Expected return on plan assets	(6,171)	(5,969)	(5,265)
Net amortization and deferral	1,648	2,526	1,773
Net pension benefit or post-retirement costs	2,156	2,496	1,516
Post-retirement Benefits
Pension Plans, Postretirement and Other Employee Benefits
Service cost	88	102	107
Interest cost	789	740	753
Expected return on plan assets	0	0	(1)
Unrecognized net (gain)/loss	(29)	0	0
Net pension benefit or post-retirement costs	$ 848	$ 842	$ 859