Pension and Retirement Plans - Components of Net Periodic Benefit Cost and Amounts Recognized in Other Comprehensive Income/ (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Amounts recognized in other comprehensive income/( loss)
Total recognized in other comprehensive income/( loss)	$ (864)	$ (124)	$ (6,545)
Pension Plan
Net periodic benefit cost
Interest cost	1,680	1,557
Expected return on assets	(2,721)	(2,891)
Amortization of prior service credit	(4)	(4)
Amortization of net actuarial loss/(gain)	154	106
Net periodic benefit cost	(891)	(1,232)
Amounts recognized in other comprehensive income/( loss)
Net actuarial loss/(gain)	(1,563)	512
Amortization of prior service credit	4	4
Amortization of net actuarial gain	(154)	(106)
Total recognized in other comprehensive income/( loss)	(1,713)	410
Total recognized in net periodic benefit cost and other comprehensive income/( loss)	(2,604)	(822)
Supplemental Retirement Plan
Net periodic benefit cost
Service cost	283	354
Interest cost	349	309
Amortization of net actuarial loss/(gain)		4
Net periodic benefit cost	632	667
Amounts recognized in other comprehensive income/( loss)
Net actuarial loss/(gain)	770	(499)
Amortization of prior service credit		(4)
Total recognized in other comprehensive income/( loss)	770	(503)
Total recognized in net periodic benefit cost and other comprehensive income/( loss)	1,402	164
Postretirement Healthcare Plan
Net periodic benefit cost
Service cost	25	23
Interest cost	25	22
Amortization of net actuarial loss/(gain)	(3)
Net periodic benefit cost	47	45
Amounts recognized in other comprehensive income/( loss)
Net actuarial loss/(gain)	76	(30)
Amortization of net actuarial gain	3
Total recognized in other comprehensive income/( loss)	79	(30)
Total recognized in net periodic benefit cost and other comprehensive income/( loss)	$ 126	$ 15