Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Cash [Abstract]
Interest paid, net of capitalized interest	$ 31.4	[1]	$ 23.7	[1]
Income taxes paid, net of refunds	0.8		0.1
Non-cash Investing and Financing balance sheet movements [Abstract]
Deadstock commodity inventories transferred to property, plant and equipment	0		1.1
Accrued dividends on unvested equity awards	0.5		0.2
Receivables from equity issuances	24.8		5.9
Capital expenditure accruals	30.9		22.7
Transfers from materials and supplies inventory to property, plant and equipment	0.6		0.4
Change in ARO liability and property, plant and equipment due to revised future ARO cash flow estimate	3.7		2.1
Interest capitalized on major projects	$ 2.4		$ 6.9

[1] Interest capitalized on major projects was $2.4 million and $6.9 million for the three months ended March 31, 2015 and 2014.