Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Supplemental Cash Flow Information [Abstract]
Cash Paid for Interest – Net of Amounts Capitalized	$ 18.3	$ 19.7
Recognition of Right-of-use Assets and Lease Liabilities	0.0	34.0
Noncash Investing and Financing Activities [Abstract]
Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment	4.7	(1.1)
Capitalized Asset Retirement Costs	1.6	1.6
AFUDC–Equity	$ 0.5	$ 0.6