BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, plant and equipment
Property, plant and equipment	$ 6,073,143	$ 5,713,912
Accumulated depreciation and amortization	1,789,431	1,706,327
Net property, plant and equipment	4,283,712	4,007,585
Current assets
Cash and cash equivalents	21,323	14,413
Accounts receivable, net	295,421	298,768
Materials and supplies	44,333	39,672
Natural gas in storage	107,295	130,154
Regulatory assets	54,420	88,180
Other current assets	20,495	17,807
Total current assets	543,287	588,994
Goodwill and other assets
Regulatory assets	437,479	405,189
Goodwill	157,953	157,953
Other assets	46,211	47,157
Total goodwill and other assets	641,643	610,299
Total assets	5,468,642	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 52,564,902 shares at December 31, 2018; issued 52,598,005 shares and outstanding 52,312,516 shares at December 31, 2017	526	526
Paid-in capital	1,727,492	1,737,551
Retained earnings	320,869	246,121
Accumulated other comprehensive income (loss)	(4,086)	(5,493)
Treasury stock, at cost: 33,103 shares at December 31, 2018 and 285,489 shares at December 31, 2017	(2,145)	(18,496)
Total equity	2,042,656	1,960,209
Long-term debt, excluding current maturities, and net of issuance costs of $11,457 and $8,033, respectively	1,285,483	1,193,257
Total equity and long-term debt	3,328,139	3,153,466
Current liabilities
Notes payable	299,500	357,215
Accounts payable	174,510	143,681
Accrued interest	18,924	18,776
Accrued taxes other than income	47,640	41,324
Accrued liabilities	30,294	30,058
Customer deposits	61,183	60,811
Regulatory liabilities	48,394	9,438
Other current liabilities	18,446	12,027
Total current liabilities	698,891	673,330
Deferred credits and other liabilities [Abstract]
Deferred income taxes	652,426	599,945
Regulatory Liability, Noncurrent	520,866	519,421
Employee benefit obligations	178,720	172,938
Other deferred credits	89,600	87,778
Total deferred credits and other liabilities	1,441,612	1,380,082
Commitments and contingencies
Total liabilities and equity	$ 5,468,642	$ 5,206,878