Employee Benefit Plans, Components of Net Periodic Benefit Cost (Details) - Defined Benefit Pension Plans Combined [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 849	$ 1,015
Interest cost	1,645	1,515
Expected return on plan assets	(2,733)	(2,793)
Amortization of loss	421	406
Amortization of prior service credit	(13)	(13)
Rate-regulated adjustment	2,131	2,170
Net periodic benefit cost	2,300	$ 2,300
Change in defined benefit plan regulatory asset from pension contribution greater (less) than net periodic benefit cost	(2,131)
Amortization of Regulatory Assets to be Reclassified into Net Periodic Benefit Cost [Abstract]
Net loss	353
Net prior service credit	(13)
Total amortization of regulatory assets to be reclassified into net periodic benefit cost during the next fiscal year	$ 340