Derivative Instruments Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk (Details) number in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Financial Derivative, Nonmonetary Notional Amount	350,250	297,925	350,250	297,925	287,475
Physical Derivative, Nonmonetary Notional Amount	296,250	241,150	296,250	241,150	420,980
Notional Amount of Derivatives [Abstract]
Derivative, Notional Amount	$ 7,920	$ 10,844	$ 7,920	$ 10,844	$ 12,230
Cost of Revenue	62,176	58,280	$ 187,881	213,481
General Discussion of Derivative Instruments and Hedging Activities [Abstract]
Derivative, Gain on Derivative		4,300		12,800
Document Period End Date			Jun. 30, 2015
Derivative, Loss on Derivative	7,900		$ 22,000
Unrealized loss On Derivatives	(14,200)		(14,200)
Derivative Asset, Fair Value, Net	1,100	11,500	1,100	11,500	200
Derivative Liability, Fair Value, Net	14,800	800	14,800	800	33,400
Derivative Fair Value, Net	(13,700)	10,700	(13,700)	10,700	$ (33,200)
Moody's, A3 Rating [Member]
Derivative [Line Items]
With Adequate Assurance Calls	0		0
Notional Amount of Derivatives [Abstract]
Without Adequate Assurance Calls	0		0
Moody's, Baa1 Rating [Member]
Derivative [Line Items]
With Adequate Assurance Calls	0		0
Notional Amount of Derivatives [Abstract]
Without Adequate Assurance Calls	0		0
Moody's, Baa2 Rating [Member]
Derivative [Line Items]
With Adequate Assurance Calls	0		0
Notional Amount of Derivatives [Abstract]
Without Adequate Assurance Calls	0		0
Moody's, Baa3 Rating [Member]
Derivative [Line Items]
With Adequate Assurance Calls	0		0
Notional Amount of Derivatives [Abstract]
Without Adequate Assurance Calls	0		0
Speculative [Member]
Derivative [Line Items]
With Adequate Assurance Calls	12,709		12,709
Notional Amount of Derivatives [Abstract]
Without Adequate Assurance Calls	8,767		8,767
Natural Gas Commodity [Member]
Notional Amount of Derivatives [Abstract]
Less:Amounts deferred to regulatory accounts on balance sheet	(9,618)	5,223	13,447	(10,652)
Total gain (loss) in pre-tax earnings	(214)	(156)	8	(119)
Natural Gas Commodity [Member] | Utility Segment [Member]
Notional Amount of Derivatives [Abstract]
Cost of Revenue	10,020	(5,379)	(13,461)	10,533
Natural Gas Commodity [Member] | Gas Storage Segment [Member]
Notional Amount of Derivatives [Abstract]
Cost of Revenue	(616)	0	22	0
Foreign Currency [Member]
Notional Amount of Derivatives [Abstract]
Less:Amounts deferred to regulatory accounts on balance sheet	(478)	(454)	263	(179)
Total gain (loss) in pre-tax earnings	0	0	0	0
Foreign Currency [Member] | Utility Segment [Member]
Notional Amount of Derivatives [Abstract]
Cost of Revenue	478	454	(263)	179
Foreign Currency [Member] | Gas Storage Segment [Member]
Notional Amount of Derivatives [Abstract]
Cost of Revenue	$ 0	$ 0	$ 0	$ 0