STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income	$ 66,578,000	$ 66,483,000	$ 62,236,000
Provided by Operating Activities:
Depreciation and Amortization	58,668,000	52,155,000	48,261,000
Provision for Losses on Accounts Receivable	14,689,000	9,417,000	4,232,000
CIP Receivable/Payable	(7,324,000)	15,226,000	21,160,000
Deferred Gas Costs - Net of Recoveries	28,648,000	(44,976,000)	5,473,000
Deferred SBC Costs - Net of Recoveries	9,557,000	11,048,000	2,393,000
Environmental Remediation Costs - Net of Recoveries	(22,058,000)	(8,248,000)	(438,000)
Deferred and Noncurrent Income Taxes and Credits - Net	39,148,000	32,193,000	31,940,000
Gas Plant Cost of Removal	(5,096,000)	(4,848,000)	(6,092,000)
Pension Contribution	(12,020,000)	0	(9,100,000)
Changes in:
Accounts Receivable	8,597,000	(14,667,000)	(20,574,000)
Inventories	11,198,000	(3,820,000)	(7,153,000)
Prepaid and Accrued Taxes - Net	(7,129,000)	(6,508,000)	9,456,000
Other Prepayments and Current Assets	(9,717,000)	131,000	(476,000)
Gas Purchases Payable	(13,423,000)	(1,873,000)	9,306,000
Accounts Payable and Other Accrued Liabilities	(2,264,000)	11,302,000	(5,107,000)
Other Assets	4,996,000	(7,481,000)	(7,323,000)
Other Liabilities	1,576,000	(2,006,000)	10,565,000
Net Cash Provided by Operating Activities	164,624,000	103,528,000	148,759,000
Cash Flows from Investing Activities:
Capital Expenditures	(207,785,000)	(200,008,000)	(161,498,000)
Note Receivable	(9,916,000)	0	0
Net Sale (Purchase) of Restricted Investments in Margin Accounts	1,091,000	(7,281,000)	588,000
Net Sale of Restricted Investments from Escrowed Loan Proceeds	101,000	0	0
Investment in Long-Term Receivables	(19,033,000)	(13,024,000)	(7,182,000)
Proceeds from Long-Term Receivables	8,769,000	6,544,000	5,764,000
Net Cash Used in Investing Activities	(226,773,000)	(213,769,000)	(162,328,000)
Cash Flows from Financing Activities:
Net Borrowings from (Repayments of) Short-Term Credit Facilities	33,000,000	35,900,000	(36,600,000)
Proceeds from Issuance of Long-Term Debt	80,000,000	89,000,000	50,000,000
Principal Repayments of Long-Term Debt	(11,009,000)	(21,000,000)	(25,000,000)
Payments for Issuance of Long-Term Debt	(9,000)	(627,000)	(411,000)
Dividends on Common Stock	(40,764,000)	(18,201,000)	0
Additional Investment by Shareholder	0	25,000,000	25,000,000
Tax Deficiency from Restricted Stock Plan	(72,000)	(73,000)	(78,000)
Net Cash Provided by Financing Activities	61,146,000	109,999,000	12,911,000
Net Decrease in Cash and Cash Equivalents	(1,003,000)	(242,000)	(658,000)
Cash and Cash Equivalents at Beginning of Period	1,778,000	2,020,000	2,678,000
Cash and Cash Equivalents at End of Period	775,000	1,778,000	2,020,000
Supplemental Disclosures of Cash Flow Information
Interest (Net of Amounts Capitalized)	19,373,000	17,832,000	12,234,000
Income Taxes (Net of Refunds)	(1,665,000)	(7,690,000)	(5,056,000)
Supplemental Disclosures of Noncash Investing Activities
Property and equipment acquired on account but not paid at year-end	$ 24,857,000	$ 17,551,000	$ 20,055,000