Pension and Postretirement Benefits Other Than Pension Plans - Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Plan [Member] | Qualified Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	$ 878	$ 803	$ 1,756	$ 1,614
Expected return on plan assets	(943)	(1,070)	(1,887)	(2,172)
Amortization of net actuarial loss	286	264	573	519
Total net periodic benefit cost (credit)	221	(3)	442	(39)
Postretirement Health Coverage [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	25	24	50	48
Amortization of net actuarial loss	62	65	123	129
Amortization of prior service credit	(329)	(329)	(657)	(657)
Total net periodic benefit cost (credit)	$ (242)	$ (240)	$ (484)	$ (480)