STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net income	$ 186,749	$ 172,234	$ 162,995
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	180,395	160,086	151,889
Deferred income taxes	13,307	53,242	92,393
Share-based compensation expense	9,314	8,195	8,876
Provision for doubtful accounts	8,994	8,506	7,323
Changes in assets and liabilities:
Accounts receivable	26,415	(5,159)	(15,147)
Materials and supplies	(11,399)	(4,661)	(5,588)
Natural gas in storage	3,036	22,859	(4,722)
Asset removal costs	(47,784)	(52,855)	(52,376)
Accounts payable	(59,293)	36,885	1,945
Accrued taxes other than income	316	6,316	(1,247)
Customer deposits	(3,196)	372	(398)
Regulatory assets and liabilities	28,203	109,437	29,250
Employee benefit obligation	(35,401)	(50,100)	(118,095)
Other assets and liabilities	10,689	2,337	(3,298)
Cash provided by operating activities	310,345	467,694	253,800
Investing activities
Capital expenditures	(417,322)	(394,450)	(356,361)
Payments to Acquire Assets, Investing Activities	(7,009)	0	0
Payments for (Proceeds from) Other Investing Activities	0	0	618
Proceeds from Sale of Other Assets, Investing Activities	1,399	0	0
Cash used in investing activities	(422,932)	(394,450)	(355,743)
Financing activities
Borrowings (repayment) on notes payable, net	217,000	(57,715)	212,215
Repurchase of common stock	0	0	(17,512)
Proceeds from Issuance of Long-term Debt	0	395,648	0
Payments of Financing Costs	0	4,324	0
Issuance of common stock	5,116	4,803	4,457
Repayments of Long-term Debt	0	(300,000)	0
Dividends paid	(105,424)	(96,594)	(87,951)
Tax withholdings related to net share settlements of stock compensation	(7,575)	(8,152)	(9,516)
Cash provided by (used in) financing activities	109,117	(66,334)	101,693
Change in cash and cash equivalents	(3,470)	6,910	(250)
Cash and cash equivalents at beginning of period	21,323	14,413	14,663
Cash and cash equivalents at end of period	17,853	21,323	14,413
Supplemental cash flow information:
Interest Paid, Excluding Capitalized Interest, Operating Activities	61,160	49,371	44,436
Cash received for income taxes, net	$ 30,152	$ 800	$ (1,389)