STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues [Abstract]
Revenues	$ 1,933,200	$ 1,918,000	$ 1,851,000
Operating expenses
Electric fuel and purchased power	1,043,500	1,055,300	1,035,000
Operating and maintenance expenses	282,700	285,400	265,500
Demand side management program expenses	17,700	15,500	16,000
Depreciation and amortization	209,600	193,900	162,400
Taxes (other than income taxes)	68,000	67,000	60,800
Total operating expenses	1,621,500	1,617,100	1,539,700
Operating income	311,700	300,900	311,300
Other income (expense), net	(3,000)	(1,800)	(3,900)
Allowance for funds used during construction — equity	19,100	9,300	10,000
Interest charges and financing costs
Interest charges — includes other financing costs of $2.9, $2.5 and $3.1, respectively	84,500	86,200	88,700
Public Utilities, Allowance For Funds Used During Construction, Capitalized Cost Of Debt	(8,900)	(5,400)	(5,600)
Total interest charges and financing costs	75,600	80,800	83,100
Income before income taxes	252,200	227,600	234,300
Income taxes	38,900	68,400	82,100
Net income	$ 213,300	$ 159,200	$ 152,200