Employee Benefit Plans Employee Benefit Plans 3 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Amounts recognized on the balance sheets:
Non-current liabilities				$ 57,695	$ 49,856
Pension Benefits
Amounts recognized on the balance sheets:
Non-current assets				0	0
Current liabilities				0	0
Non-current liabilities				34,042	29,492
Net amount recognized				(34,042)	(29,492)
Amounts recognized in regulatory assets consist of:
Prior service cost (credit)				0	0
Net (gain) loss				(32,761)	(25,828)
Regulatory assets (liabilities)	$ 25,828	$ 21,970	$ 21,970	32,761	25,828
Unfunded accrued pension cost				1,281	3,664
Net liability (asset) recognized				34,042	29,492
Changes in plan assets and benefit obligations recognized in regulatory assets:
Regulatory asset at beginning of year	25,828	21,970
Net loss (gain)	(7,856)	(4,818)
Amortization of prior service (cost) credit	0	(49)
Amortization of net gain (loss)	(923)	(911)
Total change in regulatory asset	6,933	3,858
Regulatory asset (liability) at end of year	32,761	25,828	21,970
Net periodic pension costs	4,121	4,126	6,075
Change in regulatory asset	6,933	3,858
Total recognized in net periodic pension cost and regulatory asset (liability)	11,054	7,984
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:
Prior service (cost) credit				0	0
Net gain (loss)	(1,378)	(835)
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation				207,690	180,364
Accumulated benefit obligation				190,438	165,998
Fair value of plan assets				$ 173,648	$ 150,872
Weighted-average assumptions used to determine benefit obligations at December 31:
Discount rate				3.76%	4.44%
Post-Retirement Medical Benefits
Amounts recognized on the balance sheets:
Non-current assets				$ 2,562	$ 1,736
Current liabilities				0	0
Non-current liabilities				0	0
Net amount recognized				2,562	1,736
Amounts recognized in regulatory assets consist of:
Prior service cost (credit)				0	0
Net (gain) loss				5,650	5,515
Regulatory assets (liabilities)	(5,515)	(5,606)	(5,606)	(5,650)	(5,515)
Unfunded accrued pension cost				3,088	3,779
Net liability (asset) recognized				(2,562)	(1,736)
Changes in plan assets and benefit obligations recognized in regulatory assets:
Regulatory asset at beginning of year	(5,515)	(5,606)
Net loss (gain)	910	644
Amortization of prior service (cost) credit	0	34
Amortization of net gain (loss)	775	701
Total change in regulatory asset	(135)	91
Regulatory asset (liability) at end of year	(5,650)	(5,515)	(5,606)
Net periodic pension costs	(690)	(606)	$ (234)
Change in regulatory asset	(135)	91
Total recognized in net periodic pension cost and regulatory asset (liability)	(825)	(515)
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:
Prior service (cost) credit				0	0
Net gain (loss)	$ 727	$ 679
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation				8,491	8,802
Fair value of plan assets				$ 11,053	$ 10,538
Weighted-average assumptions used to determine benefit obligations at December 31:
Discount rate				3.52%	3.97%
Minimum | Pension Benefits
Weighted-average assumptions used to determine benefit obligations at December 31:
Rate of compensation increase				3.00%
Maximum | Pension Benefits
Weighted-average assumptions used to determine benefit obligations at December 31:
Rate of compensation increase				8.00%