Consolidated Statement of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating Activities [Abstract]
Net Income	$ 122.7	$ 92.3
Allowance for Funds Used During Construction – Equity	(2.6)	(5.9)
Income from Equity Investments – Net of Dividends	(3.7)	(3.0)
Gain on Sales of Investments and Property, Plant and Equipment	(0.2)	(0.2)
Depreciation Expense	120.7	99.5
Amortization of Power Purchase Agreements	(17.1)	(9.0)
Amortization of Other Intangible Assets and Other Assets	5.0	0.8
Deferred Income Tax Expense	26.5	25.2
Share-Based Compensation Expense	2.1	1.7
ESOP Compensation Expense	7.3	6.7
Defined Benefit Pension and Postretirement Benefit Expense	11.5	9.6
Bad Debt Expense	0.8	1.1
Changes in Operating Assets and Liabilities [Abstract]
Accounts Receivable	11.6	16.9
Inventories	(24.1)	(9.2)
Prepayments and Other	(0.4)	8.8
Accounts Payable	0.0	(1.2)
Other Current Liabilities	(0.8)	(12.8)
Changes in Regulatory and Other Non-Current Assets	(6.2)	(13.0)
Changes in Regulatory and Other Non-Current Liabilities	1.5	3.8
Cash from Operating Activities	254.6	212.1
Investing Activities [Abstract]
Proceeds from Sale of Available-for-sale Securities	0.7	3.3
Payments for Purchase of Available-for-sale Securities	(1.1)	(4.3)
Acquisitions of Subsidiaries – Net of Cash Acquired	(324.8)	(23.1)
Investment in ATC	(1.2)	(3.1)
Changes to Other Investments	0.0	31.1
Additions to Property, Plant and Equipment	(208.2)	(467.8)
Cash in Escrow for Acquisition	0.0	5.4
Proceeds from Sale of Property, Plant and Equipment	0.3	0.0
Cash for Investing Activities	(534.3)	(458.5)
Financing Activities [Abstract]
Proceeds from Issuance of Common Stock	155.2	128.9
Proceeds from Issuance of Long-Term Debt	240.0	375.0
Changes in Restricted Cash	2.2	(1.4)
Changes in Notes Payable	(3.7)	2.7
Repayments of Long-Term Debt	(81.8)	(134.1)
Acquisition of Non-Controlling Interest	0.0	(6.0)
Debt Issuance Costs	(1.0)	(3.1)
Dividends on Common Stock	(74.0)	(62.4)
Cash from Financing Activities	236.9	299.6
Change in Cash and Cash Equivalents	(42.8)	53.2
Cash and Cash Equivalents at Beginning of Period	145.8	97.3
Cash and Cash Equivalents at End of Period	$ 103.0	$ 150.5