BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Property, plant and equipment
Property, plant and equipment	$ 5,545,302	$ 5,404,168
Accumulated depreciation and amortization	1,710,509	1,672,548
Net property, plant and equipment	3,834,793	3,731,620
Current assets
Cash and cash equivalents	5,113	14,663
Accounts receivable, net	152,278	290,944
Materials and supplies	36,876	34,084
Natural gas in storage	114,996	125,432
Regulatory assets	82,302	83,146
Other current assets	20,014	20,654
Total current assets	411,579	568,923
Goodwill and other assets
Regulatory assets	421,094	440,522
Goodwill	157,953	157,953
Other assets	47,711	43,773
Total goodwill and other assets	626,758	642,248
Total assets	4,873,130	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 52,267,296 shares at June 30, 2017; issued 52,598,005 and outstanding 52,283,260 at December 31, 2016	526	526
Paid-in Capital	1,734,083	1,749,574
Retained earnings	224,570	161,021
Accumulated other comprehensive income (loss)	(4,457)	(4,715)
Treasury stock, at cost: 330,709 shares at June 30, 2017 and 314,745 shares at December 31, 2016	21,426	18,126
Total equity	1,933,296	1,888,280
Long-term debt, excluding current maturities and net issuance costs of $8,445 and $8,851, respectively	1,192,848	1,192,446
Total equity and long-term debt	3,126,144	3,080,726
Current liabilities
Notes payable	79,000	145,000
Accounts payable	60,458	131,988
Accrued interest	18,958	18,854
Accrued taxes other than income	33,562	42,571
Accrued liabilities	16,202	22,931
Customer deposits	60,523	61,209
Other current liabilities	23,982	21,380
Total current liabilities	292,685	443,933
Deferred credits and other liabilities [Abstract]
Deferred income taxes	1,077,992	1,038,568
Employee benefit obligations	296,881	303,507
Other deferred credits	79,428	76,057
Total deferred credits and other liabilities	1,454,301	1,418,132
Commitments and contingencies
Total liabilities and equity	$ 4,873,130	$ 4,942,791