STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net income	$ 172,234	$ 162,995	$ 140,095
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	160,086	151,889	143,829
Deferred income taxes	53,242	92,393	86,788
Share-based compensation expense	8,195	8,876	11,219
Provision for doubtful accounts	8,506	7,323	5,427
Changes in assets and liabilities:
Accounts receivable	(5,159)	(15,147)	(80,028)
Materials and supplies	(4,661)	(5,588)	(759)
Income tax receivable	0	0	37,480
Natural gas in storage	22,859	(4,722)	16,721
Asset removal costs	(52,855)	(52,376)	(53,430)
Accounts payable	36,885	1,945	27,596
Accrued interest	148	(78)	(19)
Accrued taxes other than income	6,316	(1,247)	5,322
Accrued liabilities	236	7,127	(8,539)
Customer deposits	372	(398)	884
Regulatory assets and liabilities	109,437	29,250	(49,472)
Employee benefit obligation	(50,100)	(118,095)	(25,666)
Other assets and liabilities	1,953	(10,347)	33,141
Cash provided by operating activities	467,694	253,800	290,589
Investing activities
Capital expenditures	(394,450)	(356,361)	(309,071)
Other	0	618	492
Cash used in investing activities	(394,450)	(355,743)	(308,579)
Financing activities
Borrowings (repayment) on notes payable, net	(57,715)	212,215	132,500
Repurchase of common stock	0	(17,512)	(24,066)
Proceeds from Issuance of Long-term Debt	395,648	0	0
Payments of Financing Costs	4,324	0	0
Issuance of common stock	4,803	4,457	4,017
Repayments of Long-term Debt	(300,000)	0	0
Dividends paid	(96,594)	(87,951)	(73,209)
Tax withholdings related to net share settlements of stock compensation	(8,152)	(9,516)	(9,022)
Cash provided by (used in) financing activities	(66,334)	101,693	30,220
Change in cash and cash equivalents	6,910	(250)	12,230
Cash and cash equivalents at beginning of period	14,413	14,663	2,433
Cash and cash equivalents at end of period	21,323	14,413	14,663
Supplemental cash flow information:
Cash paid for interest, net of amounts capitalized	49,371	44,436	42,129
Cash received for income taxes, net	$ 800	$ (1,389)	$ (35,702)