Consolidated Statement of Cash Flows (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Operating Activities [Abstract]
Net Income	$ 71.7	$ 68.2
Allowance for Funds Used During Construction - Equity	(3.4)	(3.4)
Income from Equity Investments, Net of Dividends	(3.1)	(2.7)
Gain on Sale of Assets	(0.2)	0
Gain on Sale of Investments	(2.2)	0
Depreciation Expense	86.3	74.4
Amortization of Debt Issuance Costs	0.8	0.7
Deferred Income Tax Expense	20.4	23.4
Share-Based Compensation Expense	1.8	1.7
ESOP Compensation Expense	6.0	5.5
Defined Benefit Pension and Postretirement Benefit Expense	16.8	20.6
Bad Debt Expense	0.9	0.9
Changes in Operating Assets and Liabilities [Abstract]
Accounts Receivable	8.2	5.6
Inventories	5.1	(7.6)
Prepayments and Other	4.4	3.3
Accounts Payable	3.9	(1.3)
Other Current Liabilities	(8.8)	7.4
Cash Contributions to Defined Benefit Pension and Other Postretirement Benefit Plans	(10.8)	0
Changes in Regulatory and Other Non-Current Assets	(13.5)	(5.0)
Changes in Regulatory and Other Non-Current Liabilities	4.6	3.8
Cash from Operating Activities	188.9	195.5
Investing Activities [Abstract]
Proceeds from Sale of Available-for-sale Securities	15.0	1.2
Payments for Purchase of Available-for-sale Securities	(2.8)	(1.5)
Investment in ATC	(2.3)	(3.9)
Changes to Other Investments	(10.5)	(5.5)
Additions to Property, Plant and Equipment	(195.3)	(331.9)
Proceeds from Sale of Assets	0.9	0
Cash for Investing Activities	(195.0)	(341.6)
Financing Activities [Abstract]
Proceeds from Issuance of Common Stock	63.3	52.1
Proceeds from Issuance of Long-Term Debt	150.0	175.6
Changes in Notes Payable	1.2	(0.8)
Payments for Long-Term Debt	(66.2)	(24.1)
Debt Issuance Costs	(1.3)	(1.3)
Dividends on Common Stock	(57.2)	(52.2)
Cash from Financing Activities	89.8	149.3
Change in Cash and Cash Equivalents	83.7	3.2
Cash and Cash Equivalents at Beginning of Period	80.8	101.1
Cash and Cash Equivalents at End of Period	$ 164.5	$ 104.3