BALANCE SHEETS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Document Fiscal Year Focus	2018
Property, plant and equipment
Property, plant and equipment	$ 5,964,287	$ 5,713,912
Accumulated depreciation and amortization	1,768,381	1,706,327
Net property, plant and equipment	4,195,906	4,007,585
Current assets
Cash and cash equivalents	12,430	14,413
Accounts receivable, net	132,436	298,768
Materials and supplies	40,363	39,672
Natural gas in storage	126,481	130,154
Regulatory assets	49,039	88,180
Other current assets	13,762	17,807
Total current assets	374,511	588,994
Goodwill and other assets
Regulatory assets	375,059	405,189
Goodwill	157,953	157,953
Other assets	49,528	47,157
Total goodwill and other assets	582,540	610,299
Total assets	5,152,957	5,206,878
Equity and long-term debt
Common stock, $0.01 par value: authorized 250,000,000 shares; issued 52,598,005 shares and outstanding 52,516,828 shares at June 30, 2018; issued 52,598,005 and outstanding 52,312,516 at December 31, 2017	526	526
Paid-in Capital	1,725,361	1,737,551
Retained earnings	300,547	246,121
Accumulated other comprehensive income (loss)	(5,167)	(5,493)
Treasury stock, at cost: 81,177 shares at June 30, 2018 and 285,489 shares at December 31, 2017	(4,643)	(18,496)
Total equity	2,016,624	1,960,209
Long-term debt, excluding current maturities and net issuance costs of $7,614 and $8,033, respectively	893,880	1,193,257
Total equity and long-term debt	2,910,504	3,153,466
Long-term Debt, Current Maturities	300,008	8
Current liabilities
Notes payable	276,000	357,215
Accounts payable	68,332	143,681
Accrued interest	7,867	18,776
Accrued taxes other than income	48,760	41,324
Accrued liabilities	23,968	30,058
Customer deposits	61,569	60,811
Regulatory Liability, Current	41,665	9,438
Other current liabilities	7,858	12,019
Total current liabilities	836,027	673,330
Deferred credits and other liabilities [Abstract]
Deferred income taxes	634,650	599,945
Regulatory Liability, Noncurrent	521,717	519,421
Employee benefit obligations	155,443	172,938
Other deferred credits	94,616	87,778
Total deferred credits and other liabilities	1,406,426	1,380,082
Commitments and contingencies
Total liabilities and equity	$ 5,152,957	5,206,878
Over-recovered purchased-gas costs [Member]
Current liabilities
Regulatory Liability, Current		9,434
Deferred credits and other liabilities [Abstract]
Regulatory Liability, Noncurrent		$ 0