Pension and Postretirement Benefits Other Than Pension Plans - Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Plan [Member] | Qualified Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	$ 878	$ 811
Expected return on plan assets	(944)	(1,102)
Amortization of net actuarial loss	287	255
Total net periodic benefit cost (credit)	221	(36)
Postretirement Health Coverage [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	25	24
Amortization of net actuarial loss	61	64
Amortization of prior service credit	(328)	(328)
Total net periodic benefit cost (credit)	$ (242)	$ (240)