Employee Benefit Plans, Components of Net Periodic Benefit Cost (Details) - Defined Benefit Pension Plans Combined [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 1,080	$ 1,018	$ 1,166
Interest cost	1,592	1,599	1,515
Expected return on plan assets	(2,395)	(2,233)	(2,229)
Amortization of loss	493	561	705
Amortization of prior service credit	(13)	(13)	(13)
Rate-regulated adjustment	1,543	1,368	1,156
Net periodic benefit cost	2,300	$ 2,300	$ 2,300
Change in defined benefit plan regulatory asset from pension contribution greater (less) than net periodic benefit cost	(1,543)
Regulatory Assets to be Reclassified into Net Periodic Benefit Cost over Next Fiscal Year [Abstract]
Net loss	453
Net prior service credit	(13)
Regulatory assets to be reclassified into net periodic benefit cost during the next fiscal year	$ 440