Consolidated Statement of Cash Flows - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Operating Activities [Abstract]
Net Income	$ 85.9	$ 71.2
AFUDC – Equity	(0.4)	(1.2)
Income from Equity Investments – Net of Dividends	(2.5)	(2.9)
Change in Fair Value of Contingent Consideration	(0.4)	0.0
Depreciation Expense	97.9	94.2
Amortization of PSAs	(11.8)	(11.1)
Amortization of Other Intangible Assets and Other Assets	5.3	5.0
Deferred Income Tax Expense	20.2	13.8
Share-Based and ESOP Compensation Expense	3.3	2.2
Defined Benefit Pension and Postretirement Benefit Expense	5.0	2.6
Bad Debt Expense	0.0	1.1
Changes in Operating Assets and Liabilities [Abstract]
Accounts Receivable	2.4	6.5
Inventories	0.8	6.7
Prepayments and Other	4.3	(0.8)
Accounts Payable	(12.6)	1.3
Other Current Liabilities	(12.8)	(18.5)
Cash Contributions to Defined Benefit Pension Plans	(1.7)	0.0
Changes in Regulatory and Other Non-Current Assets	6.7	(21.0)
Changes in Regulatory and Other Non-Current Liabilities	(5.2)	(2.9)
Cash from Operating Activities	184.4	146.2
Investing Activities [Abstract]
Proceeds from Sale of Available-for-sale Securities	1.0	1.4
Payments for Purchase of Available-for-sale Securities	(1.6)	(1.2)
Investment in ATC	(5.0)	(1.6)
Changes to Other Investments	1.3	2.1
Additions to Property, Plant and Equipment	(81.1)	(74.8)
Proceeds from Sale of Property, Plant and Equipment	0.7	0.2
Cash for Investing Activities	(84.7)	(73.9)
Financing Activities [Abstract]
Proceeds from Issuance of Common Stock	74.4	15.2
Proceeds from Issuance of Long-Term Debt	86.2	2.2
Changes in Restricted Cash	(1.5)	(2.0)
Changes in Notes Payable	0.0	(0.7)
Repayments of Long-Term Debt	(127.0)	(32.1)
Acquisition of Non-Controlling Interest	0.0	(8.0)
Acquisition-Related Contingent Consideration Payments	(19.7)	(0.7)
Dividends on Common Stock	(54.0)	(51.2)
Other Financing Activities	(1.4)	(0.1)
Cash for Financing Activities	(43.0)	(77.4)
Change in Cash and Cash Equivalents	56.7	(5.1)
Cash and Cash Equivalents at Beginning of Period	27.5	97.0
Cash and Cash Equivalents at End of Period	$ 84.2	$ 91.9