STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenues	$ 886,936	$ 849,869	$ 917,525
Energy expenses:
Fuel	173,738	188,400	251,005
Purchased and interchanged power	59,727	53,545	64,804
Cost of services, energy services	233,465	241,945	315,809
Operating revenues net of energy expenses	653,471	607,924	601,716
Other operating expenses:
Other operations	242,014	242,950	238,832
Maintenance	66,746	65,223	65,629
Depreciation and amortization	84,317	89,824	83,342
Taxes other than income taxes	65,533	63,736	62,750
Utilities operating expense	458,610	461,733	450,553
Operating income	194,861	146,191	151,163
Other income (deductions):
Allowance for equity funds used during construction	7,023	10,639	14,662
Investment and interest income, net	14,083	17,508	13,633
Miscellaneous non-operating income	1,292	2,062	4,075
Miscellaneous non-operating deductions	(3,699)	(4,328)	(4,199)
Nonoperating income (expense)	18,699	25,881	28,171
Interest charges (credits):
Interest on long-term debt and revolving credit facility	71,544	65,851	59,028
Other interest	1,303	1,313	1,250
Capitalized interest	(4,990)	(4,968)	(5,092)
Allowance for borrowed funds used during construction	(4,983)	(6,937)	(8,368)
Interest expense	62,874	55,259	46,818
Income before income taxes	150,686	116,813	132,516
Income tax expense	53,918	34,895	41,088
Net income	$ 96,768	$ 81,918	$ 91,428
Basic earnings per share	$ 2.390	$ 2.030	$ 2.270
Diluted earnings per share	2.390	2.030	2.270
Dividends declared per share of common stock	$ 1.225	$ 1.165	$ 1.105
Weighted average number of shares outstanding	40,350,688	40,274,986	40,190,991
Weighted average number of shares and dilutive potential shares outstanding	40,408,033	40,308,562	40,211,717