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Divestitures and Acquisitions (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2017	Jan. 01, 2017
Disposal Group, Including Discontinued Operation, Additional Disclosures [Abstract]
Disposal Group, Including Discontinued Operation, Cash Purchase Price					$ 38,300
Disposal Group, Including Discontinued Operation, Working Capital Adjustment					109,000
Disposal Group, Including Discontinued Operation, Consideration					147,300
Disposal Group, Including Discontinued Operation, Consideration Placed in Escrow					$ 7,000
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax, Per Diluted Share				$ 0.03
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income tax expense		$ 0	$ 6,841
Net income from discontinued operations		0	10,994
Summary of Cash Flow Hedge Activity [Abstract]
Gain (Loss) on Discontinuation of Cash Flow Hedge		$ 0	10,579
Subsequent Event [Member]
Subsequent Event [Line Items]
Subsequent Event, Description	Regulators in our other services areas, including Texas, Mississippi and Louisiana, have also taken action in response to the TCJA: • On January 23, 2018, the Railroad Commission of Texas directed the Commission Staff to develop recommendations to ensure that, beginning January 1, 2018, all gas utility customers in Texas receive the full benefit of the TCJA. •On January 26, 2018, the Mississippi Public Service Commission (MPSC) entered an order requiring each utility to file within thirty days a detailed description identifying how the TCJA will be reflected in the formula rate plan or other rate structures under which the utility operates. •On January 31, 2018, Louisiana Public Service Commission (LPSC) directed utilities to file reports on February 14, 2018, regarding savings for ratepayers as a result of the new federal tax laws. The LPSC is also considering an accounting order to direct the utilities to track and record the impacts of the TCJA and a rule making docket to address the TCJA. In January 2018, $3.0 million of this escrowed amount was released and received by the Company.
Disposal Group, Including Discontinued Operation, Consideration Placed in Escrow Released	$ 3,000
Natural Gas Marketing Segment [Member] \| Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Operating revenues			303,474
Purchased gas cost			277,554
Operating expenses			7,874
Operating income			18,046
Other nonoperating expense			(211)
Income from discontinued operations before income taxes			17,835
Income tax expense			6,841
Net income from discontinued operations			10,994
Discontinued Operation, Alternative Cash Flow Information [Abstract]
Depreciation and Amortization, Discontinued Operations			185
Capital Expenditure, Discontinued Operations			0
Noncash Change in Commodity Contract Cash Flow Hedges			(8,165)
Discontinued Operation, Additional Disclosures [Abstract]
Gain on Discontinuation of Cash Flow Hedge			10,600
Gain (Loss) On Hedge Ineffectiveness			3,400
Effect of Fair Value Hedges on Results of Operations [Abstract]
Commodity contracts			(9,567)
Fair value adjustment for natural gas inventory designated as the hedged item			12,858
Total (increase) decrease to purchased gas cost			3,291
Basis ineffectiveness			597
Timing ineffectiveness			(3,888)
Summary of Cash Flow Hedge Activity [Abstract]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts			(2,612)
Gain (loss) arising from ineffective portion of commodity contracts			111
Gain (Loss) on Discontinuation of Cash Flow Hedge			10,579
Total impact on purchased gas cost			8,078
Gain (Loss) On Derivative Instruments Not Designated Hedges Net Pretax			$ 6,800