Pension and Other Post-Retirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension benefits
Components of net periodic benefit cost
Service cost	$ 2,098	$ 1,274
Interest cost	1,714	1,827
Expected return on plan assets	(3,091)	(2,759)
Amortization of prior service cost	(581)	(581)
Amortization of the net loss	924	855
Net periodic cost	1,064	616
Pension benefits | Minimum
Components of net periodic benefit cost
Estimated contribution for the current year	2,000
Pension benefits | Maximum
Components of net periodic benefit cost
Estimated contribution for the current year	4,000
Other post-retirement benefits
Components of net periodic benefit cost
Service cost	0	0
Interest cost	7	11
Expected return on plan assets	0	0
Amortization of prior service cost	0	0
Amortization of the net loss	5	17
Net periodic cost	$ 12	$ 28