Pension and Post-Retirement Benefits - Net Pension and Post-Retirement Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits
Pension Plans, Postretirement and Other Employee Benefits
Service cost	$ 653	$ 586
Interest cost	989	2,584
Expected return on plan assets	(1,421)	(3,184)
Net amortization and deferral	894	624
Net benefit costs	1,115	610
Post-retirement Benefits
Pension Plans, Postretirement and Other Employee Benefits
Service cost	25	24
Interest cost	129	196
Net benefit costs	$ 154	$ 220