BALANCE SHEETS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Document Fiscal Year Focus	2019
Property, plant and equipment
Property, plant and equipment	$ 6,148,394	$ 6,073,143
Accumulated depreciation and amortization	1,816,480	1,789,431
Net property, plant and equipment	4,331,914	4,283,712
Current assets
Cash and cash equivalents	19,628	21,323
Accounts receivable, net	338,082	295,421
Materials and supplies	46,895	44,333
Natural gas in storage	51,475	107,295
Regulatory assets	38,760	54,420
Other current assets	21,430	20,495
Total current assets	516,270	543,287
Goodwill and other assets
Regulatory assets	429,518	437,479
Goodwill	157,953	157,953
Other assets	88,637	46,211
Total goodwill and other assets	676,108	641,643
Total assets	5,524,292	5,468,642
Equity and long-term debt
Common stock, $0.01 par value: authorized 250,000,000 shares; issued and outstanding 52,686,634 shares at March 31, 2019; issued 52,598,005 and outstanding 52,564,902 shares at December 31, 2018	527	526
Paid-in Capital	1,720,220	1,727,492
Retained earnings	389,177	320,869
Accumulated other comprehensive income (loss)	(5,144)	(4,086)
Treasury stock, at cost: 33,103 shares at December 31, 2018	0	(2,145)
Total equity	2,104,780	2,042,656
Long-term debt, excluding current maturities and net issuance costs of $11,368 and $11,457, respectively	1,285,587	1,285,483
Total equity and long-term debt	3,390,367	3,328,139
Current liabilities
Notes payable	295,500	299,500
Accounts payable	119,629	174,510
Accrued taxes other than income	51,903	47,640
Regulatory Liability, Current	54,791	48,394
Customer deposits	62,401	61,183
Other current liabilities	73,967	67,664
Total current liabilities	658,191	698,891
Deferred credits and other liabilities [Abstract]
Deferred income taxes	666,438	652,426
Regulatory Liability, Noncurrent	511,743	520,866
Employee benefit obligations	173,296	178,720
Other deferred credits	124,257	89,600
Total deferred credits and other liabilities	1,475,734	1,441,612
Commitments and contingencies
Total liabilities and equity	5,524,292	5,468,642
Over-recovered purchased-gas costs [Member]
Current liabilities
Regulatory Liability, Current	14,197	13,668
Deferred credits and other liabilities [Abstract]
Regulatory Liability, Noncurrent	$ 0	$ 0