Employee Benefit Plans Employee Benefit Plans 14 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized on the balance sheets:
Non-current liabilities				$ (57,695)	$ (49,856)
SERP
Amounts recognized on the balance sheets:
Current liabilities				(409)	(419)
Non-current liabilities				(23,653)	(20,364)
Net amount recognized				(24,062)	(20,783)
Amounts recognized in regulatory assets consist of:
Prior service cost (credit)				0	11
Net loss				7,556	6,463
Regulatory assets (liabilities)	$ 6,474	$ 3,452	$ 3,452	7,556	6,474	$ 3,452
Unfunded accrued pension cost				16,506	14,309
Net liability (asset) recognized				24,062	20,783
Changes in plan assets and benefit obligations recognized in regulatory assets:
Regulatory asset at beginning of year	6,474	3,452
Net (gain) loss	1,872	3,339
Amortization of prior service (cost) credit	(12)	(25)
Amortization of net gain (loss)	(778)	(292)
Change in regulatory asset	1,082	3,022
Regulatory asset (liability) at end of year	7,556	6,474	3,452
Net periodic pension costs	2,612	1,859	$ 2,015
Change in regulatory asset	1,082	3,022
Total recognized in net periodic pension cost and regulatory asset (liability)	3,694	4,881
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:
Initial net asset (obligation)	0	0
Prior service cost	0	(11)
Net gain (loss)	$ (1,049)	$ (777)
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation				24,062	20,783	$ 16,317
Accumulated benefit obligation				20,742	17,144
Fair value of plan assets				$ 0	$ 0
Weighted-average assumptions used to determine benefit obligations at December 31:
Discount rate				3.72%	4.34%
Rate of compensation increase				4.00%	4.00%