Derivative Instruments Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Derivative [Line Items]
Financial Derivative, Nonmonetary Notional Amount	229,925	295,125	287,000
Physical Derivative, Nonmonetary Notional Amount	250,250	875,150	420,980
Derivative, Notional Amount	$ 8,690	$ 5,590	$ 12,230
Cost of Revenue	125,705	155,201
General Discussion of Derivative Instruments and Hedging Activities [Abstract]
Derivative, Gain on Derivative		8,500
Document Period End Date	Mar. 31, 2015
Derivative, Loss on Derivative	14,100
Unrealized loss On Derivatives	23,100
Derivative Asset, Fair Value, Net	600	16,600	200
Derivative Liability, Fair Value, Net	24,200	1,000	33,400
Derivative Fair Value, Net	23,600	15,600	33,200
Moody's, A3 Rating [Member]
Derivative [Line Items]
With Adequate Assurance Calls	0
Without Adequate Assurance Calls	0
Moody's, Baa1 Rating [Member]
Derivative [Line Items]
With Adequate Assurance Calls	0
Without Adequate Assurance Calls	0
Moody's, Baa2 Rating [Member]
Derivative [Line Items]
With Adequate Assurance Calls	0
Without Adequate Assurance Calls	0
Moody's, Baa3 Rating [Member]
Derivative [Line Items]
With Adequate Assurance Calls	0
Without Adequate Assurance Calls	0
Speculative [Member]
Derivative [Line Items]
With Adequate Assurance Calls	20,683
Without Adequate Assurance Calls	15,773
Natural Gas Commodity [Member]
Derivative [Line Items]
Less:Amounts deferred to regulatory accounts on balance sheet	23,065	(15,875)
Total gain (loss) in pre-tax earnings	222	37
Natural Gas Commodity [Member] | Utility Segment [Member]
Derivative [Line Items]
Cost of Revenue	(23,481)	15,912
Natural Gas Commodity [Member] | Gas Storage Segment [Member]
Derivative [Line Items]
Cost of Revenue	638	0
Foreign Currency [Member]
Derivative [Line Items]
Less:Amounts deferred to regulatory accounts on balance sheet	741	(275)
Total gain (loss) in pre-tax earnings	0	0
Foreign Currency [Member] | Utility Segment [Member]
Derivative [Line Items]
Cost of Revenue	(741)	275
Foreign Currency [Member] | Gas Storage Segment [Member]
Derivative [Line Items]
Cost of Revenue	$ 0	$ 0