Condensed Consolidated Statements of Income and Comprehensive Income (Unaudited) - USD ($) shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues, net	$ 449,000,000	$ 428,000,000	$ 979,000,000	$ 915,000,000
Operating expenses:
Purchased power and fuel	118,000,000	126,000,000	259,000,000	275,000,000
Generation, transmission and distribution	81,000,000	64,000,000	162,000,000	130,000,000
Administrative and other	65,000,000	61,000,000	133,000,000	122,000,000
Depreciation and amortization	86,000,000	83,000,000	170,000,000	165,000,000
Taxes other than income taxes	31,000,000	30,000,000	64,000,000	60,000,000
Total operating expenses	381,000,000	364,000,000	788,000,000	752,000,000
Income from operations	68,000,000	64,000,000	191,000,000	163,000,000
Interest expense, net	30,000,000	27,000,000	60,000,000	54,000,000
Other income:
Allowance for equity funds used during construction	3,000,000	8,000,000	5,000,000	15,000,000
Miscellaneous income, net	1,000,000	1,000,000	2,000,000	0
Other income, net	4,000,000	9,000,000	7,000,000	15,000,000
Income before income tax expense	42,000,000	46,000,000	138,000,000	124,000,000
Income tax expense	10,000,000	9,000,000	33,000,000	26,000,000
Net income	32,000,000	37,000,000	105,000,000	98,000,000
Other comprehensive income	1,000,000	0	0	0
Comprehensive income	$ 33,000,000	$ 37,000,000	$ 105,000,000	$ 98,000,000
Weighted-average shares outstanding-basic and diluted (in thousands)	89,063	88,902	89,033	88,867
Earnings per share-basic and diluted	$ 0.36	$ 0.42	$ 1.18	$ 1.10
Dividends declared per common share	$ 0.34	$ 0.32	$ 0.66	$ 0.62