Consolidated Statement of Cash Flows (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Operating Activities [Abstract]
Net Income	$ 40.1	$ 33.8
Allowance for Funds Used During Construction – Equity	(0.9)	(1.8)
Income from Equity Investments, Net of Dividends	(0.8)	(0.8)
Depreciation Expense	38.4	32.2
Amortization of Intangible Assets and Other Assets	0.8	0.3
Amortization of Power Purchase Agreements	(4.9)	(2.3)
Deferred Income Tax Expense	6.1	8.8
Share-Based Compensation Expense	0.6	0.8
ESOP Compensation Expense	2.4	2.2
Defined Benefit Pension and Postretirement Benefit Expense	3.8	3.2
Bad Debt Expense	0.2	0.2
Changes in Operating Assets and Liabilities [Abstract]
Accounts Receivable	7.8	2.9
Inventories	(8.8)	(6.5)
Prepayments and Other	(0.7)	2.2
Accounts Payable	(14.0)	0.1
Other Current Liabilities	(1.7)	1.6
Changes in Regulatory and Other Non-Current Assets	(4.1)	(4.0)
Changes in Regulatory and Other Non-Current Liabilities	7.5	2.0
Cash from Operating Activities	71.8	74.9
Investing Activities [Abstract]
Proceeds from Sale of Available-for-sale Securities	0.2	0.6
Payments for Purchase of Available-for-sale Securities	(0.4)	(0.7)
Acquisitions of Subsidiaries – Net of Cash Acquired	(166.9)	(23.1)
Investment in ATC	(0.4)	(1.2)
Changes to Other Investments	0	30.0
Additions to Property, Plant and Equipment	(88.2)	(216.2)
Construction Costs for Development Project	(0.2)	0
Cash in Escrow for Acquisition	0	6.0
Cash for Investing Activities	(255.9)	(204.6)
Financing Activities [Abstract]
Proceeds from Issuance of Common Stock	141.5	24.8
Proceeds from Issuance of Long-Term Debt	0	100.0
Changes in Restricted Cash	(0.8)	0
Changes in Notes Payable	(3.4)	0
Reductions of Long-Term Debt	(2.0)	(19.8)
Acquisition of Non-Controlling Interest	0	(6.0)
Debt Issuance Costs	0	(0.9)
Dividends on Common Stock	(24.9)	(21.1)
Cash from Financing Activities	110.4	77.0
Change in Cash and Cash Equivalents	(73.7)	(52.7)
Cash and Cash Equivalents at Beginning of Period	145.8	97.3
Cash and Cash Equivalents at End of Period	$ 72.1	$ 44.6