Consolidated Balance Sheet (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Current Assets [Abstract]
Cash and Cash Equivalents	$ 164.5	$ 80.8
Accounts Receivable (Less Allowance of $1.1 and $1.0)	79.9	89.0
Inventories	64.7	69.8
Prepayments and Other	29.2	33.6
Deferred Income Taxes	30.7	0
Total Current Assets	369.0	273.2
Property, Plant and Equipment - Net	2,456.1	2,347.6
Regulatory Assets	340.3	340.3
Investment in ATC	112.7	107.3
Other Investments	142.9	143.5
Other Non-Current Assets	42.9	41.5
Total Assets	3,463.9	3,253.4
Current Liabilities [Abstract]
Accounts Payable	84.3	90.5
Accrued Taxes	26.1	30.2
Accrued Interest	14.9	15.6
Long-Term Debt Due Within One Year	37.7	84.5
Notes Payable	1.2	0
Other	59.3	62.6
Total Current Liabilities	223.5	283.4
Long-Term Debt	1,064.2	933.6
Deferred Income Taxes	474.9	423.8
Regulatory Liabilities	68.7	60.1
Defined Benefit Pension and Other Postretirement Benefit Plans	216.0	228.2
Other Non-Current Liabilities	129.1	123.3
Total Liabilities	2,176.4	2,052.4
Commitments, Guarantees and Contingencies (Note 14)
Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 40.7 and 39.4 Shares Outstanding	849.8	784.7
Unearned ESOP Shares	(15.1)	(21.3)
Accumulated Other Comprehensive Loss	(21.3)	(22.0)
Retained Earnings	474.1	459.6
Total Equity	1,287.5	1,201.0
Total Liabilities and Equity	$ 3,463.9	$ 3,253.4