Derivative Instruments Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Unrealized Gain/Loss [Abstract]
Operating revenues	$ 230,718,000	$ 93,128,000	$ 138,280,000	$ 261,665,000	$ 240,283,000	$ 87,199,000	$ 133,169,000	$ 293,386,000	$ 723,791,000	$ 754,037,000	$ 758,518,000
General Discussion of Derivative Instruments and Hedging Activities [Abstract]
Derivative, Loss on Derivative									37,700,000
Derivative, Gain on Derivative										10,500,000
Unrealized loss On Derivatives									23,200,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	2,700,000				200,000				2,700,000	200,000
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	25,500,000				33,400,000				$ 25,500,000	33,400,000
Maturity Of Credit Risk Derivatives									Mar. 01, 2018
Derivative Fair Value Of Derivative Net	$ (22,800,000)				$ (33,200,000)				$ (22,800,000)	$ (33,200,000)
Notional Disclosures [Abstract]
Financial Derivative, Nonmonetary Notional Amount	346,875,000				287,475,000				346,875,000	287,475,000
Derivative, Nonmonetary Notional Amount	404,645,000				420,980,000				404,645,000	420,980,000
Notional Amount of Foreign Currency Derivatives	$ 9,025,000				$ 12,230,000				$ 9,025,000	$ 12,230,000
Natural Gas Commodity [Member]
Unrealized Gain/Loss [Abstract]
Cost of sales									(22,600,000)	(32,784,000)
Operating revenues									226,000	0
Less:Amounts deferred to regulatory accounts on balance sheet									22,434,000	32,782,000
Total Impact On Earnings									60,000	(2,000)
Foreign Currency [Member]
Unrealized Gain/Loss [Abstract]
Cost of sales									(419,000)	(382,000)
Operating revenues									0	0
Less:Amounts deferred to regulatory accounts on balance sheet									419,000	382,000
Total Impact On Earnings									$ 0	$ 0