CONDENSED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Revenues from contracts with customers	$ 477.4	$ 477.9
Other revenues	12.6	14.8
Operating revenues	490.0	492.7
COST OF SALES	212.6	210.5
OPERATING EXPENSES
Other operation and maintenance	120.3	113.6
Depreciation and amortization	82.4	78.8
Taxes other than income	24.4	22.7
Operating expenses	227.1	215.1
OPERATING INCOME	50.3	67.1
OTHER INCOME (EXPENSE)
Allowance for equity funds used during construction	1.5	7.0
Other net periodic benefit income (expense)	0.4	(4.8)
Other income	1.4	3.1
Other expense	(0.7)	(0.8)
Net other income	2.6	4.5
INTEREST EXPENSE
Interest on long-term debt	32.6	39.6
Allowance for borrowed funds used during construction	(1.0)	(3.7)
Interest on short-term debt and other interest charges	0.8	1.4
Interest expense	32.4	37.3
INCOME BEFORE TAXES	20.5	34.3
INCOME TAX EXPENSE	0.9	3.0
NET INCOME	19.6	31.3
Other comprehensive income, net of tax	0.0	0.0
COMPREHENSIVE INCOME	$ 19.6	$ 31.3