BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property, plant and equipment
Property, plant and equipment	$ 5,643,136	$ 5,404,168
Accumulated depreciation and amortization	1,730,857	1,672,548
Net property, plant and equipment	3,912,279	3,731,620
Current assets
Cash and cash equivalents	6,872	14,663
Accounts receivable, net	127,689	290,944
Materials and supplies	38,789	34,084
Natural gas in storage	157,641	125,432
Regulatory assets	99,548	83,146
Other current assets	15,014	20,654
Total current assets	445,553	568,923
Goodwill and other assets
Regulatory assets	411,653	440,522
Goodwill	157,953	157,953
Other assets	43,625	43,773
Total goodwill and other assets	613,231	642,248
Total assets	4,971,063	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 52,273,444 shares at September 30, 2017; issued 52,598,005 and outstanding 52,283,260 at December 31, 2016	526	526
Paid-in Capital	1,735,638	1,749,574
Retained earnings	221,185	161,021
Accumulated other comprehensive income (loss)	(4,329)	(4,715)
Treasury stock, at cost: 324,561 shares at September 30, 2017 and 314,745 shares at December 31, 2016	21,028	18,126
Total equity	1,931,992	1,888,280
Long-term debt, excluding current maturities and net issuance costs of $8,239 and $8,851, respectively	1,193,052	1,192,446
Total equity and long-term debt	3,125,044	3,080,726
Current liabilities
Notes payable	174,000	145,000
Accounts payable	68,184	131,988
Accrued interest	7,742	18,854
Accrued taxes other than income	44,658	42,571
Accrued liabilities	18,535	22,931
Customer deposits	59,643	61,209
Other current liabilities	19,466	21,380
Total current liabilities	392,228	443,933
Deferred credits and other liabilities [Abstract]
Deferred income taxes	1,089,061	1,038,568
Employee benefit obligations	282,904	303,507
Other deferred credits	81,826	76,057
Total deferred credits and other liabilities	1,453,791	1,418,132
Commitments and contingencies
Total liabilities and equity	$ 4,971,063	$ 4,942,791