Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - Supplemental Retirement Plan [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net periodic benefit cost [Abstract]
Service cost	$ 29	$ 35	$ 58	$ 70
Interest cost	457	514	952	1,075
Amortization of prior service cost	62	62	125	125
Recognized net actuarial loss	220	215	446	437
Net periodic benefit cost	$ 768	$ 826	$ 1,581	$ 1,707