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, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee Benefit Plans [Abstract]
Maximum period of eligible service	30 years
Change in Benefit Obligation [Abstract]
Pension benefit obligation, beginning of year	$ 39,469	$ 40,884
Service cost	1,018	1,166	$ 952
Interest cost	1,599	1,515	1,444
Actuarial (gain) loss	(43)	(2,873)
Benefit payments	(1,289)	(1,223)
Pension benefit obligation, end of year	40,754	39,469	40,884
Change in Plan Assets [Abstract]
Fair value of plan assets, beginning of year	31,835	30,575
Actual return on plan assets	2,621	183
Employer contributions	2,300	2,300
Benefits paid	(1,289)	(1,223)
Fair value of plan assets, end of year	35,467	31,835	30,575
Funded Status of Plans at end of year	$ (5,287)	$ (7,634)
Change in discount rate	(0.20%)	0.40%
Threshold for amortization of gains and losses	10.00%
Changes in Plan Assets and Benefit Obligations Recognized in Regulatory Assets [Abstract]
Net (gain) loss arising during the period	$ (432)	$ (826)
Recognized net actuarial loss	(561)	(705)	$ (126)
Recognized prior service credit (cost)	13	13
Total changes in regulatory asset during the year	(980)	(1,518)
Amounts Recognized in Regulatory Assets that Have Not Yet Been Recognized as Components of Net Periodic Benefit Cost [Abstract]
Net loss	9,454	10,447
Prior service (credit) cost	(114)	(127)
Regulatory asset	$ 9,340	$ 10,320