Employee Benefit Plans, Changes in Benefit Obligation and Plan Assets (Details) - Defined Benefit Pension Plans Combined [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Benefit Plans [Abstract]
Maximum period of eligible service	30 years
Change in Benefit Obligation [Abstract]
Pension benefit obligation, beginning of year	$ 40,754	$ 39,469
Service cost	1,080	1,018	$ 1,166
Interest cost	1,592	1,599	1,515
Actuarial (gain) loss	2,645	(43)
Benefit payments	(1,435)	(1,289)
Pension benefit obligation, end of year	44,636	40,754	39,469
Change in Plan Assets [Abstract]
Fair value of plan assets, beginning of year	35,467	31,835
Actual return on plan assets	5,107	2,621
Employer contributions	2,300	2,300
Benefits paid	(1,435)	(1,289)
Fair value of plan assets, end of year	41,439	35,467	31,835
Funded Status of Plans at end of year	$ (3,197)	$ (5,287)
Change in discount rate	(0.50%)	(0.20%)
Threshold for amortization of gains and losses	10.00%
Changes in Plan Assets and Benefit Obligations Recognized in Regulatory Assets [Abstract]
Net gain arising during the period	$ (66)	$ (432)
Recognized net actuarial loss	(493)	(561)	$ (705)
Recognized prior service credit	13	13
Total changes in regulatory asset during the year	(546)	(980)
Amounts Recognized in Regulatory Assets that Have Not Yet Been Recognized as Components of Net Periodic Benefit Cost [Abstract]
Net loss	8,895	9,454
Prior service credit	(101)	(114)
Regulatory asset	$ 8,794	$ 9,340