Significant Accounting Policies 606 Revenue Adoption (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Item Effected [Line Items]
Operating revenues	$ 91,239	$ 86,213	$ 479,441	$ 516,413
Total operating expenses	97,350	90,753	409,532	425,437
Revenue taxes	3,522	0	20,731
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Item Effected [Line Items]
Operating revenues	87,700		458,700
Total operating expenses	93,900		388,800
Calculated under Revenue Guidance in Effect after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Item Effected [Line Items]
Operating revenues	91,200		479,400
Total operating expenses	97,400		409,500
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Item Effected [Line Items]
Operating revenues	3,500		20,700
Total operating expenses	3,500		20,700
Total Revenue Taxes [Member]
Item Effected [Line Items]
Revenue taxes	$ 3,500	3,700	$ 20,700	23,000
Revenue taxes in operating revenues [Member]
Item Effected [Line Items]
Revenue taxes		2,300		13,300
Revenue taxes on Balance Sheet [Member]
Item Effected [Line Items]
Revenue taxes		$ 1,400		$ 9,700