Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - Supplemental Retirement Plan [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net periodic benefit cost [Abstract]
Service cost	$ 29	$ 36	$ 87	$ 106
Interest cost	457	514	1,409	1,589
Amortization of prior service cost	63	63	188	188
Recognized net actuarial loss	218	214	664	651
Net periodic benefit cost	$ 767	$ 827	$ 2,348	$ 2,534