Derivative Instruments Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Nov. 01, 2016	Dec. 31, 2015 USD ($)	Nov. 01, 2015
Derivative [Line Items]
Target Hedge for 2015-16 gas year							75.00%
Financial Derivative, Nonmonetary Notional Amount	537,100,000	416,075,000	537,100,000	416,075,000		346,875,000
Derivative, Nonmonetary Notional Amount	621,230,000	521,350,000	621,230,000	521,350,000		404,645,000
Notional Amount of Foreign Currency Derivatives	$ 8,404	$ 8,023	$ 8,404	$ 8,023		$ 9,025
Target Hedge for 2015-16 gas year					75.00%
Operating revenues	87,727	93,128	442,439	493,073
Derivative, Loss on Derivative	975	2,300	24,062	24,300
Unrealized Gain On Derivatives			2,682
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	4,058	3,100	4,058	3,100		2,700
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	5,066	25,300	5,066	25,300		25,500
Derivative Fair Value Of Derivative Net	1,008	22,200	1,008	22,200		$ 22,800
Natural Gas Commodity [Member]
Derivative [Line Items]
Cost of sales	(8,045)	(8,415)	5,562	(21,876)
Operating revenues	(110)	33	(266)	55
Less:Amounts deferred to regulatory accounts on balance sheet	8,118	8,391	(5,385)	21,838
Total Impact On Earnings	(37)	9	(89)	17
Foreign Currency [Member]
Derivative [Line Items]
Cost of sales	(52)	(150)	5	(413)
Operating revenues	0	0	0	0
Less:Amounts deferred to regulatory accounts on balance sheet	52	150	(5)	413
Total Impact On Earnings	$ 0	$ 0	$ 0	$ 0