Consolidated Statement of Cash Flows (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Operating Activities [Abstract]
Net Income	$ 38.8	$ 54.0
Allowance for Funds Used During Construction	(1.9)	(1.1)
Income from Equity Investments, Net of Dividends	(1.7)	(0.9)
Gain on Sale of Assets	0	(0.7)
Depreciation Expense	49.4	44.4
Amortization of Debt Issuance Costs	0.5	0.5
Deferred Income Tax Expense	13.1	11.8
Share-Based Compensation Expense	1.2	1.1
ESOP Compensation Expense	3.7	3.6
Defined Benefit Pension and Postretirement Benefit Expense	13.8	12.3
Bad Debt Expense	0.5	0.5
Changes in Operating Assets and Liabilities [Abstract]
Accounts Receivable	12.4	27.2
Inventories	(0.2)	(1.3)
Prepayments and Other	1.6	8.0
Accounts Payable	(8.2)	(17.7)
Other Current Liabilities	0	(10.1)
Cash Contributions to Defined Benefit Pension and Other Postretirement Benefit Plans	0	(10.9)
Changes in Regulatory and Other Non-Current Assets	(1.8)	(1.2)
Changes in Regulatory and Other Non-Current Liabilities	6.9	10.5
Cash from Operating Activities	128.1	130.0
Investing Activities [Abstract]
Proceeds from Sale of Available-for-sale Securities	1.0	7.2
Payments for Purchase of Available-for-sale Securities	(1.0)	(1.2)
Investment in ATC	(2.0)	(1.4)
Changes to Other Investments	(3.2)	(1.4)
Additions to Property, Plant and Equipment	(221.8)	(91.6)
Proceeds from Sale of Assets	0	1.4
Cash for Investing Activities	(227.0)	(87.0)
Financing Activities [Abstract]
Proceeds from Issuance of Common Stock	30.5	22.9
Proceeds from Issuance of Long-Term Debt	15.6	0
Proceeds (Payments) from (for) Notes Payable	(1.1)	1.5
Reductions of Long-term Debt	(3.1)	(1.5)
Dividends on Common Stock	(35.5)	(31.4)
Cash from (for) Financing Activities	6.4	(8.5)
Change in Cash and Cash Equivalents	(92.5)	34.5
Cash and Cash Equivalents at Beginning of Period	101.1	44.9
Cash and Cash Equivalents at End of Period	$ 8.6	$ 79.4