Consolidated Statement of Cash Flows (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Operating Activities [Abstract]
Net Income	$ 33.8	$ 32.5
Allowance for Funds Used During Construction - Equity	(1.8)	(1.1)
Income from Equity Investments, Net of Dividends	(0.8)	(1.3)
Gain on Sale of Assets / Investments	0	(1.0)
Depreciation Expense	32.2	28.2
Amortization of Debt Issuance Costs	0.3	0.3
Deferred Income Tax Expense	8.8	7.3
Share-Based Compensation Expense	0.8	0.6
ESOP Compensation Expense	2.2	2.0
Defined Benefit Pension and Postretirement Benefit Expense	3.2	5.6
Bad Debt Expense	0.2	0.1
Changes in Operating Assets and Liabilities [Abstract]
Accounts Receivable	2.9	2.7
Inventories	(6.5)	5.5
Prepayments and Other	2.2	5.5
Accounts Payable	0.1	(11.0)
Other Current Liabilities	1.6	(0.7)
Cash Contributions to Defined Benefit Pension and Other Postretirement Benefit Plans	0	(10.8)
Changes in Regulatory and Other Non-Current Assets	(4.0)	(1.9)
Changes in Regulatory and Other Non-Current Liabilities	(0.3)	(4.3)
Cash from Operating Activities	74.9	58.2
Investing Activities [Abstract]
Proceeds from Sale of Available-for-sale Securities	0.6	7.9
Payments for Purchase of Available-for-sale Securities	(0.7)	(1.2)
Investment in ATC	(1.2)	(0.4)
Changes to Other Investments	30.0	(1.0)
Additions to Property, Plant and Equipment	(216.2)	(69.8)
Acquisition – Net of Cash Acquired	(23.1)	0
Changes in Restricted Cash	6.0	0
Proceeds from Sale of Assets	0	0.6
Cash for Investing Activities	(204.6)	(63.9)
Financing Activities [Abstract]
Proceeds from Issuance of Common Stock	24.8	23.4
Proceeds from Issuance of Long-Term Debt	100.0	0
Reductions of Long-Term Debt	(19.8)	(1.7)
Debt Issuance Costs	(0.9)	0
Acquisition of Non-Controlling Interest	(6.0)	0
Dividends on Common Stock	(21.1)	(19.5)
Cash from Financing Activities	77.0	2.2
Change in Cash and Cash Equivalents	(52.7)	(3.5)
Cash and Cash Equivalents at Beginning of Period	97.3	80.8
Cash and Cash Equivalents at End of Period	$ 44.6	$ 77.3