Consolidated Statement of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating Activities [Abstract]
Net Income	$ 113.0	$ 130.8
AFUDC – Equity	(0.9)	(0.7)
Income from Equity Investments – Net of Dividends	(1.3)	(3.8)
Depreciation Expense	149.3	147.4
Amortization of PSAs	(17.9)	(17.7)
Amortization of Other Intangible Assets and Other Assets	7.8	7.6
Deferred Income Tax Expense	(9.1)	34.3
Share-Based and ESOP Compensation Expense	5.1	5.0
Defined Benefit Pension and Postretirement Benefit Expense	6.4	7.6
Provision for Interim Rate Refund	13.0	0.0
Provision for Tax Reform Refund	9.2	0.0
Bad Debt Expense	1.5	0.5
Changes in Operating Assets and Liabilities [Abstract]
Accounts Receivable	3.9	(9.6)
Inventories	(12.6)	5.3
Prepayments and Other	5.7	2.1
Accounts Payable	9.9	(2.6)
Other Current Liabilities	21.2	2.7
Cash Contributions to Defined Benefit Pension Plans	(15.0)	(1.7)
Changes in Regulatory and Other Non-Current Assets	7.2	23.9
Changes in Regulatory and Other Non-Current Liabilities	1.7	(23.9)
Cash from Operating Activities	298.1	307.2
Investing Activities [Abstract]
Proceeds from Sale of Available-for-sale Securities	9.3	5.2
Payments for Purchase of Available-for-sale Securities	(12.5)	(5.9)
Payments to Acquire Businesses, Net of Cash Acquired	0.0	17.4
Investment in ATC	(5.0)	(6.6)
Additions to Property, Plant and Equipment	(201.0)	(130.3)
Payments for (Proceeds from) Other Investing Activities	1.4	3.3
Cash for Investing Activities	(207.8)	(151.7)
Financing Activities [Abstract]
Proceeds from Issuance of Common Stock	14.6	80.5
Proceeds from Issuance of Long-term Debt	72.7	131.5
Repayments of Long-Term Debt	(59.5)	(183.6)
Acquisition-Related Contingent Consideration Payments	0.0	(19.7)
Dividends on Common Stock	(86.2)	(81.4)
Other Financing Activities	(0.5)	(1.6)
Cash for Financing Activities	(58.9)	(74.3)
Change in Cash, Cash Equivalents and Restricted Cash	31.4	81.2
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 141.5	$ 119.5