BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, plant and equipment
Property, plant and equipment	$ 5,404,168	$ 5,132,682
Accumulated depreciation and amortization	1,672,548	1,620,771
Net property, plant and equipment	3,731,620	3,511,911
Current assets
Cash and cash equivalents	14,663	2,433
Accounts receivable, net	290,944	216,343
Materials and supplies	34,084	33,325
Income tax receivable	1,397	38,877
Natural gas in storage	125,432	142,153
Regulatory assets	83,146	32,925
Other current assets	19,257	16,789
Total current assets	568,923	482,845
Goodwill and other assets
Regulatory assets	440,522	435,863
Goodwill	157,953	157,953
Other assets	43,773	46,193
Total goodwill and other assets	642,248	640,009
Total assets	4,942,791	4,634,765
Equity and long-term debt
Common stock, $0.01 par value: authorized 250,000,000 shares; issued 52,598,005 shares and outstanding 52,283,260 shares at December 31, 2016; issued 52,598,005 shares and outstanding 52,259,224 shares at December 31, 2015	526	526
Paid-in capital	1,749,574	1,764,875
Retained earnings	161,021	95,046
Accumulated other comprehensive income (loss)	(4,715)	(4,401)
Treasury stock, at cost: 338,781 shares at December 31, 2015 and 314,745 shares at December 31, 2016	(18,126)	(14,491)
Total equity	1,888,280	1,841,555
Long-term debt, excluding current maturities, and net of issuance costs of $8,851 and $9,645, respectively	1,192,446	1,191,660
Total equity and long-term debt	3,080,726	3,033,215
Current liabilities
Current maturities of long-term debt	7	7
Notes payable	145,000	12,500
Accounts payable	131,988	107,482
Accrued interest	18,854	18,873
Accrued taxes other than income	42,571	37,249
Accrued liabilities	22,931	31,470
Customer deposits	61,209	60,325
Regulatory liabilities	11,922	24,615
Other current liabilities	9,451	11,700
Total current liabilities	443,933	304,221
Deferred credits and other liabilities [Abstract]
Deferred income taxes	1,038,568	951,785
Employee benefit obligations	303,507	272,309
Other deferred credits	76,057	73,235
Total deferred credits and other liabilities	1,418,132	1,297,329
Commitments and contingencies
Total liabilities and equity	$ 4,942,791	$ 4,634,765