Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 139.7	$ 0	$ 146.4	$ 0
Cash and Cash Equivalents, Fair Value Disclosure	139.7	0
Financial liabilities
Derivative instruments - long-term	0	3.6
Fair Value, Measurements, Recurring [Member]
Financial Assets
Total financial assets	123.0	192.8
Financial liabilities
Total financial liabilities	2.5	6.2
Fair Value, Measurements, Recurring [Member] | Commodity derivative [Member]
Financial Assets
Derivative instruments - short-term	104.1	188.7
Derivative instruments - long-term	17.6	4.1
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	1.3
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	95.5	39.7
Long-term debt	3,405.7	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 derivative [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 Assets
Derivative instruments - long-term	0
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	95.5	39.7
Long-term debt	3,465.7	3,420.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Total financial assets	124.7	193.9
Financial liabilities
Total financial liabilities	4.2	7.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodity derivative [Member]
Financial Assets
Derivative instruments - short-term	105.8	189.7
Derivative instruments - long-term	17.6	4.2
Financial liabilities
Derivative instruments - short-term	1.7	1.0
Derivative instruments - long-term		0.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	1.3
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 derivative [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 Assets
Derivative instruments - long-term	0
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	(1.7)	(1.1)
Financial liabilities
Total financial liabilities	(1.7)	(1.1)
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member] | Commodity derivative [Member]
Financial Assets
Derivative instruments - short-term	(1.7)	(1.0)
Derivative instruments - long-term	0	(0.1)
Financial liabilities
Derivative instruments - short-term	(1.7)	(1.0)
Derivative instruments - long-term		(0.1)
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member] | Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	0
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		$ 0