STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues	$ 916,797	$ 886,936	$ 849,869
Energy expenses:
Fuel	185,069	173,738	188,400
Purchased and interchanged power	59,682	59,727	53,545
Cost of services, energy services	244,751	233,465	241,945
Operating revenues net of energy expenses	672,046	653,471	607,924
Other operating expenses:
Other operations	242,628	242,014	242,950
Maintenance	69,458	66,746	65,223
Depreciation and amortization	90,843	84,317	89,824
Taxes other than income taxes	70,863	65,533	63,736
Utilities operating expense	473,792	458,610	461,733
Operating income	198,254	194,861	146,191
Other income (deductions):
Allowance for equity funds used during construction	3,025	7,023	10,639
Investment and interest income, net	17,757	14,083	17,508
Miscellaneous non-operating income	715	1,292	2,062
Miscellaneous non-operating deductions	(3,125)	(3,699)	(4,328)
Nonoperating income	18,372	18,699	25,881
Interest charges (credits):
Interest on long-term debt and revolving credit facility	72,970	71,544	65,851
Other interest	2,388	1,303	1,313
Capitalized interest	(5,022)	(4,990)	(4,968)
Allowance for borrowed funds used during construction	(2,975)	(4,983)	(6,937)
Interest expense	67,361	62,874	55,259
Income before income taxes	149,265	150,686	116,813
Income tax expense	51,004	53,918	34,895
Net income	$ 98,261	$ 96,768	$ 81,918
Basic earnings per share	$ 2.420	$ 2.390	$ 2.030
Diluted earnings per share	2.420	2.390	2.030
Dividends declared per share of common stock	$ 1.315	$ 1.225	$ 1.165
Weighted average number of shares outstanding	40,414,556	40,350,688	40,274,986
Weighted average number of shares and dilutive potential shares outstanding	40,535,191	40,408,033	40,308,562