Consolidated Statement of Cash Flows (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Operating Activities [Abstract]
Net Income	$ 32.5	$ 24.4
Allowance for Funds Used During Construction - Equity	(1.1)	(0.8)
Income from Equity Investments, Net of Dividends	(1.3)	(0.7)
Gain on Sale of Assets	(0.2)	0
Gain on Sale of Investments	(0.8)	0
Depreciation Expense	28.2	24.6
Amortization of Debt Issuance Costs	0.3	0.2
Deferred Income Tax Expense	7.3	8.3
Share-Based Compensation Expense	0.6	0.7
ESOP Compensation Expense	2.0	1.8
Defined Benefit Pension and Postretirement Benefit Expense	5.6	6.9
Bad Debt Expense	0.1	0.3
Changes in Operating Assets and Liabilities [Abstract]
Accounts Receivable	2.7	2.7
Inventories	5.5	(3.1)
Prepayments and Other	5.5	4.2
Accounts Payable	(11.0)	(7.9)
Other Current Liabilities	(0.7)	(0.9)
Cash Contributions to Defined Benefit Pension and Other Postretirement Benefit Plans	(10.8)	0
Changes in Regulatory and Other Non-Current Assets	(1.9)	2.1
Changes in Regulatory and Other Non-Current Liabilities	(4.3)	5.7
Cash from Operating Activities	58.2	68.5
Investing Activities [Abstract]
Proceeds from Sale of Available-for-sale Securities	7.9	0.5
Payments for Purchase of Available-for-sale Securities	(1.2)	(0.8)
Investment in ATC	(0.4)	(0.8)
Changes to Other Investments	(1.0)	(1.5)
Additions to Property, Plant and Equipment	(69.8)	(53.9)
Proceeds from Sale of Assets	0.6	0
Cash for Investing Activities	(63.9)	(56.5)
Financing Activities [Abstract]
Proceeds from Issuance of Common Stock	23.4	23.7
Proceeds from Notes Payable	0	0.5
Payments for Long-Term Debt	(1.7)	(1.4)
Dividends on Common Stock	(19.5)	(17.7)
Cash from Financing Activities	2.2	5.1
Change in Cash and Cash Equivalents	(3.5)	17.1
Cash and Cash Equivalents at Beginning of Period	80.8	101.1
Cash and Cash Equivalents at End of Period	$ 77.3	$ 118.2