Pension and Postretirement Benefits Other Than Pension Plans - Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net settlement loss	$ 1,577	$ 1,004	$ 1,577	$ 1,004
Pension Plan [Member] | Qualified Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	798	808	2,554	2,422
Expected return on plan assets	(935)	(1,087)	(2,822)	(3,259)
Amortization of net actuarial loss	293	259	866	778
Net settlement loss	1,600	1,000	1,600	1,000
Total net periodic benefit cost (credit)	1,733	984	2,175	945
Postretirement Health Coverage [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	26	24	76	72
Amortization of net actuarial loss	55	64	178	193
Amortization of prior service credit	(329)	(329)	(986)	(986)
Total net periodic benefit cost (credit)	$ (248)	$ (241)	$ (732)	$ (721)