Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Cash Flows [Abstract]
Net income	$ 363.3	$ 359.5	$ 336.8
Reconciliation to cash provided by operating activities
Depreciation and amortization	384.4	348.1	331.6
Deferred income taxes and investment tax credits, net	(131.2)	(0.7)	109.7
Contributions and payments related to pension and OPEB plans	(5.5)	(6.3)	(8.3)
Payments for liabilities transferred to affiliates	(4.5)	(10.1)	(0.3)
Change In
Accounts receivable and unbilled revenues	60.3	34.8	(64.9)
Material, supplies, and inventories	11.6	9.3	20.3
Prepaid taxes	34.0	(28.3)	0.5
Other current assets	(5.2)	13.5	(11.8)
Accounts payable	(22.4)	13.2	45.8
Accrued taxes	(1.3)	(41.1)	12.8
Other current liabilities	(1.1)	(5.2)	12.2
Other, net	172.0	275.5	(86.4)
Net cash provided by operating activities	854.4	962.2	698.0
Investing Activities
Capital expenditures	(590.6)	(603.2)	(596.1)
Proceeds from the sale of assets	2.0	1.7	22.9
Proceeds from assets transferred to affiliates	0.1	8.8	0.0
Payments for assets transferred from affiliates	0.0	(59.8)	0.0
Other, net	11.6	12.1	5.0
Net cash used in investing activities	(576.9)	(640.4)	(568.2)
Financing Activities
Change in short-term debt	(19.4)	(76.0)	51.9
Repayment of subsidiary note to parent	0.0	0.0	(18.5)
Issuance of long-term debt	300.0	300.0	0.0
Retirement of long-term debt	(250.0)	(250.0)	0.0
Payments for finance lease obligations	(50.5)	0.0	0.0
Equity contribution from parent	105.0	28.0	75.0
Payment of dividends to parent	(360.0)	(310.0)	(240.0)
Other, net	(3.7)	(5.9)	(1.3)
Net cash used in financing activities	(278.6)	(313.9)	(132.9)
Net change in cash and cash equivalents	(1.1)	7.9	(3.1)
Cash and cash equivalents at beginning of year	20.2	12.3	15.4
Cash and cash equivalents at end of year	$ 19.1	$ 20.2	$ 12.3