Consolidated Statement of Cash Flows (USD $) In Millions	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010
Operating Activities [Abstract]
Net Income	$ 74.5	$ 61.7
Allowance for Funds Used During Construction	(1.7)	(3.4)
Income from Equity Investments, Net of Dividends	(1.9)	(2.2)
Gain on Real Estate Foreclosure	0	(0.7)
Gain on Sale of Assets	(0.9)	0
Depreciation Expense	67.1	59.8
Amortization of Debt Issuance Costs	0.7	0.7
Deferred Income Tax Expense	24.6	65.0
Share-Based Compensation Expense	1.7	1.6
ESOP Compensation Expense	5.3	5.2
Bad Debt Expense	1.0	0.8
Changes in Operating Assets and Liabilities [Abstract]
Accounts Receivable	22.8	5.6
Inventories	(9.1)	(5.8)
Prepayments and Other	5.8	(2.4)
Accounts Payable	(16.5)	3.7
Other Current Liabilities	(4.4)	(2.0)
Changes in Defined Benefit Pension and Other Postretirement Benefit Plans	(14.3)	(8.5)
Changes in Regulatory and Other Non-Current Assets	13.2	10.6
Changes in Regulatory and Other Non-Current Liabilities	17.2	(1.7)
Cash from Operating Activities	185.1	188.0
Investing Activities [Abstract]
Proceeds from Sale of Available-for-sale Securities	7.4	0.6
Payments for Purchase of Available-for-sale Securities	(1.6)	(1.8)
Investment in ATC	(2.0)	(1.2)
Changes to Other Investments	(4.1)	(2.6)
Additions to Property, Plant and Equipment	(156.8)	(172.7)
Proceeds from Sale of Assets	2.2	0
Cash for Investing Activities	(154.9)	(177.7)
Financing Activities [Abstract]
Proceeds from Issuance of Common Stock	30.1	19.0
Proceeds from Issuance of Long-Term Debt	75.0	155.0
Payments on Long-Term Debt	(2.8)	(70.2)
Debt Issuance Costs	0	(1.4)
Dividends on Common Stock	(46.9)	(45.2)
Changes in Notes Payable	4.6	(0.9)
Cash from Financing Activities	60.0	56.3
Change in Cash and Cash Equivalents	90.2	66.6
Cash and Cash Equivalents at Beginning of Period	44.9	25.7
Cash and Cash Equivalents at End of Period	$ 135.1	$ 92.3