Consolidated Statement of Cash Flows - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Operating Activities [Abstract]
Net Income	$ 62.4	$ 50.7
Allowance for Funds Used During Construction – Equity	(1.6)	(3.8)
Income from Equity Investments – Net of Dividends	(2.3)	(1.9)
Gain on Sale of Investments	(0.1)	(0.2)
Depreciation Expense	78.7	66.0
Amortization of Intangible Assets and Other Assets	2.9	0.5
Amortization of Power Purchase Agreements	(11.0)	(6.2)
Deferred Income Tax Expense	12.3	13.6
Share-Based Compensation Expense	1.3	1.4
ESOP Compensation Expense	4.9	4.5
Defined Benefit Pension and Postretirement Benefit Expense	7.7	6.4
Bad Debt Expense	0.3	0.6
Changes in Operating Assets and Liabilities [Abstract]
Accounts Receivable	17.3	21.0
Inventories	(13.4)	(12.9)
Prepayments and Other	4.2	7.0
Accounts Payable	(25.6)	(11.0)
Other Current Liabilities	47.4	(9.7)
Changes in Regulatory and Other Non-Current Assets	(9.6)	(11.5)
Changes in Regulatory and Other Non-Current Liabilities	6.5	11.6
Cash from Operating Activities	182.3	126.1
Investing Activities [Abstract]
Proceeds from Sale of Available-for-sale Securities	0.7	2.7
Payments for Purchase of Available-for-sale Securities	(0.8)	(3.4)
Acquisitions of Subsidiaries – Net of Cash Acquired	(214.4)	(23.1)
Investment in ATC	(0.8)	(2.3)
Changes to Other Investments	(0.4)	30.6
Additions to Property, Plant and Equipment	(140.5)	(333.9)
Cash in Escrow for Acquisition	(15.0)	6.0
Cash for Investing Activities	(371.2)	(323.4)
Financing Activities [Abstract]
Proceeds from Issuance of Common Stock	148.2	38.9
Proceeds from Issuance of Long-Term Debt	15.0	215.0
Changes in Restricted Cash	(2.9)	0.0
Changes in Notes Payable	(3.7)	0.0
Repayments of Long-Term Debt	(3.4)	(20.8)
Acquisition of Non-Controlling Interest	0.0	(6.0)
Debt Issuance Costs	0.0	(1.8)
Dividends on Common Stock	(49.5)	(41.7)
Cash from Financing Activities	103.7	183.6
Change in Cash and Cash Equivalents	(85.2)	(13.7)
Cash and Cash Equivalents at Beginning of Period	145.8	97.3
Cash and Cash Equivalents at End of Period	$ 60.6	$ 83.6