Other Current Assets and Other Current Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Other current assets
Prepaid expenses	$ 13.9	$ 11.9
Unrealized gains on derivatives	10.1	0.5
Deposits	3.8	4.6
Product exchanges	0.1	1.0
Other current assets	27.9	18.0
Other current liabilities
Unrealized losses on derivatives	5.6	74.7
Product exchanges		3.3
Other current liabilities	$ 5.6	$ 78.0