BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Property, plant and equipment
Property, plant and equipment	$ 5,338,591	$ 5,132,682
Accumulated depreciation and amortization	1,658,266	1,620,771
Net property, plant and equipment	3,680,325	3,511,911
Current assets
Cash and cash equivalents	4,513	2,433
Accounts receivable, net	105,060	216,343
Materials and supplies	30,098	33,325
Income tax receivable	6,952	38,877
Natural gas in storage	144,230	142,153
Regulatory assets	73,863	32,925
Other current assets	12,457	16,789
Total current assets	377,173	482,845
Goodwill and other assets
Regulatory assets	431,086	435,863
Goodwill	157,953	157,953
Other assets	47,142	46,193
Total goodwill and other assets	636,181	640,009
Total assets	4,693,679	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 52,245,273 shares at September 30, 2016; issued 52,598,005 shares and outstanding 52,259,224 shares at December 31, 2015	526	526
Paid-in Capital	1,748,965	1,764,875
Retained earnings	137,221	95,046
Accumulated other comprehensive income (loss)	(4,054)	(4,401)
Treasury stock, at cost: 352,732 shares at September 30, 2016 and 338,781 shares at December 31, 2015	(20,314)	(14,491)
Total equity	1,862,344	1,841,555
Long-term debt, excluding current maturities and net of issuance costs of $9,051 and $9,645, respectively	1,192,248	1,191,660
Total equity and long-term debt	3,054,592	3,033,215
Current liabilities
Current maturities of long-term debt	7	7
Notes payable	41,000	12,500
Accounts payable	70,562	107,482
Accrued interest	7,691	18,873
Accrued taxes other than income	39,919	37,249
Accrued liabilities	17,812	31,470
Customer deposits	60,425	60,325
Regulatory liabilities	13,204	24,615
Other current liabilities	8,212	11,700
Total current liabilities	258,832	304,221
Deferred credits and other liabilities [Abstract]
Deferred income taxes	1,011,691	951,785
Employee benefit obligations	291,230	272,309
Other deferred credits	77,334	73,235
Total deferred credits and other liabilities	1,380,255	1,297,329
Commitments and contingencies
Total liabilities and equity	$ 4,693,679	$ 4,634,765