Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Cash Paid During the Period for Interest – Net of Amounts Capitalized	$ 19.6	$ 15.3
Cash Paid During the Period for Income Taxes	0.1	0.1
Noncash Investing and Financing Activities [Abstract]
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment	(25.6)	(32.2)
Capitalized Asset Retirement Costs	3.5	1.2
AFUDC–Equity	0.9	0.9
Contingent Consideration	$ 0.0	$ 35.7