Significant Accounting Policies 606 Revenue Adoption (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Item Effected [Line Items]
Operating revenues	$ 124,567	$ 134,476	$ 388,202	$ 430,200
Revenue taxes	4,780	0	17,209	0
Total operating expenses	116,298	118,041	312,182	334,684
Revenue taxes on Balance Sheet [Member]
Item Effected [Line Items]
Revenue taxes		2,400		8,300
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Item Effected [Line Items]
Operating revenues	119,800		371,000
Total operating expenses	111,500		295,000
Calculated under Revenue Guidance in Effect after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Item Effected [Line Items]
Operating revenues	124,600		388,200
Total operating expenses	116,300		312,200
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Item Effected [Line Items]
Operating revenues	4,800		17,200
Total operating expenses	4,800		17,200
Total Revenue Taxes [Member]
Item Effected [Line Items]
Revenue taxes	$ 4,800	5,600	$ 17,200	19,300
Revenue taxes in operating revenues [Member]
Item Effected [Line Items]
Revenue taxes		$ 3,200		$ 11,000