Significant Accounting Policies 606 Revenue Adoption (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Item Effected [Line Items]
Operating revenues	$ 264,712	$ 297,323
Excise and Sales Taxes	12,429	0
Operating Costs and Expenses	197,364	219,791
Revenue taxes on Balance Sheet [Member]
Item Effected [Line Items]
Excise Taxes Collected		5,900
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Item Effected [Line Items]
Operating revenues	252,300
Operating Costs and Expenses	185,000
Calculated under Revenue Guidance in Effect after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Item Effected [Line Items]
Operating revenues	264,700
Operating Costs and Expenses	197,400
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Item Effected [Line Items]
Operating revenues	12,400
Operating Costs and Expenses	12,400
Total Revenue Taxes [Member]
Item Effected [Line Items]
Excise and Sales Taxes	12,400
Excise Taxes Collected		13,700
Revenue taxes in operating revenues [Member]
Item Effected [Line Items]
Excise Taxes Collected		$ 7,800
Gas Storage Segment [Member]
Item Effected [Line Items]
Operating revenues	$ 5,233