Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Current Assets [Abstract]
Cash and Cash Equivalents	$ 128.0	$ 98.9
Accounts Receivable (Less Allowance of $2.6 and $2.1)	129.4	135.1
Inventories – Net	154.8	95.9
Prepayments and Other	27.2	37.6
Total Current Assets	439.4	367.5
Property, Plant and Equipment – Net	3,846.5	3,822.4
Regulatory Assets	370.6	384.7
Investment in ATC	126.0	118.7
Other Investments	51.7	53.1
Goodwill and Intangible Assets – Net	224.7	225.9
Other Non-Current Assets	104.4	107.7
Total Assets	5,163.3	5,080.0
Current Liabilities [Abstract]
Accounts Payable	131.8	136.3
Accrued Taxes	53.9	50.0
Accrued Interest	15.5	17.6
Long-Term Debt Due Within One Year	56.6	64.1
Other	144.9	83.2
Total Current Liabilities	402.7	351.2
Long-Term Debt	1,461.7	1,439.2
Deferred Income Taxes	231.3	230.5
Regulatory Liabilities	512.3	532.0
Defined Benefit Pension and Other Postretirement Benefit Plans	173.6	191.8
Other Non-Current Liabilities	265.6	267.1
Total Liabilities	3,047.2	3,011.8
Commitments, Guarantees and Contingencies (Note 13)
Shareholders’ Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 51.4 and 51.1 Shares Issued and Outstanding	1,421.1	1,401.4
Accumulated Other Comprehensive Loss	(27.3)	(22.6)
Retained Earnings	722.3	689.4
Total Shareholders’ Equity	2,116.1	2,068.2
Total Liabilities and Shareholders' Equity	$ 5,163.3	$ 5,080.0