BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Property, plant and equipment
Property, plant and equipment	$ 5,132,682	$ 4,850,201
Accumulated depreciation and amortization	1,620,771	1,556,481
Net property, plant and equipment	3,511,911	3,293,720
Current assets
Cash and cash equivalents	2,433	11,943
Accounts receivable, net	216,343	326,749
Materials and supplies	33,325	27,511
Income tax receivable	38,877	43,800
Natural gas in storage	142,153	185,300
Regulatory assets	32,925	50,193
Other current assets	16,789	22,005
Total current assets	482,845	667,501
Goodwill and other assets
Regulatory assets	435,863	478,723
Goodwill	157,953	157,953
Other assets	55,838	51,313
Total goodwill and other assets	649,654	687,989
Total assets	4,644,410	4,649,210
Equity and long-term debt
Common stock, $0.01 par value: authorized 250,000,000 shares; issued 52,598,005 shares and outstanding 52,259,224 at December 31, 2015; authorized 250,000,000 shares, issued and outstanding 52,083,859 shares at December 31, 2014	526	521
Paid-in capital	1,764,875	1,758,796
Retained earnings	95,046	39,894
Accumulated other comprehensive income (loss)	(4,401)	(5,174)
Treasury stock, at cost: 338,781 shares at December 31, 2015 and no shares at December 31, 2014	(14,491)	0
Total equity	1,841,555	1,794,037
Long-term debt, excluding current maturities	1,201,305	1,201,311
Total equity and long-term debt	3,042,860	2,995,348
Current liabilities
Current maturities of long-term debt	7	6
Notes payable	12,500	42,000
Accounts payable	107,482	159,064
Accrued interest	18,873	18,872
Accrued taxes other than income	37,249	44,742
Accrued liabilities	31,470	26,019
Customer deposits	60,325	60,003
Regulatory liabilities	24,615	32,467
Other current liabilities	11,700	9,260
Total current liabilities	304,221	392,433
Deferred credits and other liabilities [Abstract]
Deferred income taxes	951,785	894,585
Employee benefit obligations	272,309	287,779
Other deferred credits	73,235	79,065
Total deferred credits and other liabilities	$ 1,297,329	$ 1,261,429
Commitments and contingencies
Total liabilities and equity	$ 4,644,410	$ 4,649,210