Derivative Instruments Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) therm	Mar. 31, 2016 USD ($) therm	Dec. 31, 2016 USD ($) therm	Nov. 01, 2016	Nov. 01, 2015
Notional Disclosures [Abstract]
Financial Derivative, Nonmonetary Notional Amount | therm	382,850,000	317,100,000	477,430,000
Derivative, Nonmonetary Notional Amount | therm	368,700,000	169,978,000	535,450,000
Notional Amount of Foreign Currency Derivatives	$ 6,629	$ 6,852	$ 7,497
Derivative [Line Items]
Target Hedge 2015-2016 Gas Year					75.00%
Target Hedge 2016-2017 Gas Year				75.00%
General Discussion of Derivative Instruments and Hedging Activities [Abstract]
Derivative, Loss on Derivative	300	15,500
Unrealized Gain (loss) On Derivatives	1,400
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	1,700	1,800	18,800
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	2,900	17,900	700
Derivative Fair Value Of Derivative Net	(1,200)	(16,100)	$ 18,100
Natural gas commodity
Unrealized Gain/Loss [Abstract]
Amounts deferred to regulatory accounts on balance sheet	13,893	7,215
Total loss in pre-tax earnings	(427)	0
Foreign exchange
Unrealized Gain/Loss [Abstract]
Amounts deferred to regulatory accounts on balance sheet	(26)	(404)
Total loss in pre-tax earnings	0	0
Benefit (expense) to cost of gas | Natural gas commodity
Unrealized Gain/Loss [Abstract]
Amounts deferred and non-deferred	(13,094)	(7,215)
Benefit (expense) to cost of gas | Foreign exchange
Unrealized Gain/Loss [Abstract]
Amounts deferred and non-deferred	26	404
Operating loss | Natural gas commodity
Unrealized Gain/Loss [Abstract]
Amounts deferred and non-deferred	(1,226)	0
Operating loss | Foreign exchange
Unrealized Gain/Loss [Abstract]
Amounts deferred and non-deferred	$ 0	$ 0