BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Property, plant and equipment
Property, plant and equipment	$ 5,468,277	$ 5,404,168
Accumulated depreciation and amortization	1,694,389	1,672,548
Net property, plant and equipment	3,773,888	3,731,620
Current assets
Cash and cash equivalents	13,206	14,663
Accounts receivable, net	249,155	290,944
Materials and supplies	34,288	34,084
Natural gas in storage	91,231	125,432
Regulatory assets	74,636	83,146
Other current assets	18,616	20,654
Total current assets	481,132	568,923
Goodwill and other assets
Regulatory assets	430,338	440,522
Goodwill	157,953	157,953
Other assets	46,563	43,773
Total goodwill and other assets	634,854	642,248
Total assets	4,889,874	4,942,791
Equity and long-term debt
Common stock, $0.01 par value: authorized 250,000,000 shares; issued 52,598,005 shares and outstanding 52,431,914 shares at March 31, 2017; issued 52,598,005 and outstanding 52,283,260 at December 31, 2016	526	526
Paid-in Capital	1,732,335	1,749,574
Retained earnings	226,195	161,021
Accumulated other comprehensive income (loss)	(4,586)	(4,715)
Treasury stock, at cost: 166,091 shares at March 31, 2017 and 314,745 shares at December 31, 2016	9,892	18,126
Total equity	1,944,578	1,888,280
Long-term debt, excluding current maturities and net issuance costs of $8,648 and $8,851, respectively	1,192,647	1,192,446
Total equity and long-term debt	3,137,225	3,080,726
Current liabilities
Notes payable	85,400	145,000
Accounts payable	76,142	131,988
Accrued interest	7,667	18,854
Accrued taxes other than income	37,431	42,571
Accrued liabilities	12,474	22,931
Customer deposits	61,945	61,209
Other current liabilities	26,884	21,380
Total current liabilities	307,943	443,933
Deferred credits and other liabilities [Abstract]
Deferred income taxes	1,065,096	1,038,568
Employee benefit obligations	300,535	303,507
Other deferred credits	79,075	76,057
Total deferred credits and other liabilities	1,444,706	1,418,132
Commitments and contingencies
Total liabilities and equity	$ 4,889,874	$ 4,942,791