Employee Benefit Plans, Components of Net Periodic Benefit Cost (Details) - Defined Benefit Pension Plans Combined [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 1,018	$ 1,166	$ 952
Interest cost	1,599	1,515	1,444
Expected return on plan assets	(2,233)	(2,229)	(1,986)
Amortization of loss	561	705	126
Amortization of prior service (credit) cost	(13)	(13)	(13)
Rate-regulated adjustment	1,368	1,156	1,659
Net periodic benefit cost	2,300	$ 2,300	$ 2,182
Change in defined benefit plan regulatory asset from pension contribution greater (less) than net periodic benefit cost	(1,368)
Regulatory Assets to be Reclassified into Net Periodic Benefit Cost over Next Fiscal Year [Abstract]
Net loss	500
Net prior service credit	(13)
Regulatory assets to be reclassified into net periodic benefit cost during the next fiscal year	$ 487