Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 123.0	$ 0	$ 0	$ 0
Cash and Cash Equivalents, Fair Value Disclosure	123.0	0
Financial Assets
Derivative Asset, Fair Value, Gross	63.8	193.9
Financial liabilities
Derivative instruments - long-term	0	3.6
Fair Value, Measurements, Recurring [Member]
Financial Assets
Total financial assets	45.2	192.8
Financial liabilities
Total financial liabilities	10.1	6.2
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	39.4	188.7
Derivative instruments - long-term	5.8	4.1
Financial liabilities
Derivative instruments - short-term	7.6	0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	2.5	2.6
Derivative instruments - long-term		3.6
Fair Value, Measurements, Recurring [Member] | Carrying Amount [Member]
Financial liabilities
Checks outstanding in excess of cash balances	1.6	39.7
Long-term debt	2,882.3	3,206.9
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial Assets
Total financial assets	0	0
Financial liabilities
Total financial liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	0	0
Derivative instruments - long-term	0	0
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Fair Value [Member]
Financial liabilities
Checks outstanding in excess of cash balances	1.6	39.7
Long-term debt	2,900.5	3,420.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Total financial assets	63.8	193.9
Financial liabilities
Total financial liabilities	28.7	7.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term		189.7
Derivative instruments - long-term		4.2
Financial liabilities
Derivative instruments - short-term	26.2	1.0
Derivative instruments - long-term		0.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	2.5	2.6
Derivative instruments - long-term		3.6
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial Assets
Total financial assets	0	0
Financial liabilities
Total financial liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	0	0
Derivative instruments - long-term	0	0
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member]
Financial Assets
Total financial assets	(18.6)	(1.1)
Financial liabilities
Total financial liabilities	(18.6)	(1.1)
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	(18.6)	(1.0)
Derivative instruments - long-term	0	(0.1)
Financial liabilities
Derivative instruments - short-term	(18.6)	(1.0)
Derivative instruments - long-term		(0.1)
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member] | Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		0
Long-term [Member] | Commodity Contract [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		4.2
Long-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodity Contract [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	5.8
Short-term [Member] | Commodity Contract [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		189.7
Short-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodity Contract [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	$ 58.0