Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Deferred Net Periodic Benefit Cost [Abstract]
Deferred pension and OPRB costs	$ 3,000		$ 3,000
Maximum PSCW approved deferred pension and OPRB costs			5,900
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	1,221	$ 1,404	2,362	$ 2,861
Interest cost	3,742	3,222	7,194	6,430
Expected return on assets	(5,922)	(6,552)	(11,469)	(13,120)
Amortization of:
Prior service credit	(31)	(11)	(59)	(22)
Actuarial loss	1,985	1,341	3,714	2,639
Net periodic benefit (credit) cost	995	(596)	1,742	(1,212)
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	334	310	553	642
Interest cost	865	648	1,442	1,306
Expected return on assets	(676)	(814)	(1,357)	(1,616)
Amortization of:
Transition obligation	1	0	2	1
Prior service credit	(662)	(667)	(1,330)	(1,334)
Actuarial loss	94	116	200	244
Net periodic benefit (credit) cost	$ (44)	$ (407)	$ (490)	$ (757)