Derivative Instruments Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Nov. 01, 2016	Nov. 01, 2015
Derivative [Line Items]
Target Hedge 2015-2016 Gas Year													75.00%
Target Hedge 2016-2017 Gas Year												75.00%
Unrealized Gain/Loss [Abstract]
Operating revenues	$ 233,528,000	$ 87,727,000	$ 99,183,000	$ 255,529,000	$ 230,718,000	$ 93,128,000	$ 138,280,000	$ 261,665,000	$ 675,967,000	$ 723,791,000	$ 754,037,000
General Discussion of Derivative Instruments and Hedging Activities [Abstract]
Derivative, Loss on Derivative									26,900,000
Derivative, Gain on Derivative										37,700,000
Unrealized loss On Derivatives									15,400,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	18,800,000				2,700,000				18,800,000	2,700,000
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	700,000				25,500,000				$ 700,000	25,500,000
Maturity Of Credit Risk Derivatives									Mar. 31, 2019
Derivative Fair Value Of Derivative Net	$ 18,100,000				$ (22,800,000)				$ 18,100,000	$ (22,800,000)
Notional Disclosures [Abstract]
Financial Derivative, Nonmonetary Notional Amount	477,430,000				346,875,000				477,430,000	346,875,000
Derivative, Nonmonetary Notional Amount	535,450,000				404,645,000				535,450,000	404,645,000
Notional Amount of Foreign Currency Derivatives	$ 7,497,000				$ 9,025,000				$ 7,497,000	$ 9,025,000
Natural Gas Commodity [Member]
Unrealized Gain/Loss [Abstract]
Cost of sales									(22,746,000)	16,469,000
Operating revenues									995,000	178,000
Less:Amounts deferred to regulatory accounts on balance sheet									(23,394,000)	16,351,000
Total Impact On Earnings									347,000	60,000
Foreign Currency [Member]
Unrealized Gain/Loss [Abstract]
Cost of sales									130,000	419,000
Operating revenues									0	0
Less:Amounts deferred to regulatory accounts on balance sheet									130,000	419,000
Total Impact On Earnings									$ 0	$ 0