Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets and liabilities measured at estimated fair value on a recurring basis
Derivative asset	$ 603.6	$ 695.3
Derivative liability	573.0	649.6
Total, inventories liability		22.9
Level 1
Assets and liabilities measured at estimated fair value on a recurring basis
Subtotal Assets	207.3	173.7
Subtotal Liabilities	339.2	306.4
Level 2
Assets and liabilities measured at estimated fair value on a recurring basis
Investments		2.2
Subtotal Assets	397.3	519.6
Inventories liability	5.8	22.9
Subtotal Liabilities	239.3	364.8
Level 3
Assets and liabilities measured at estimated fair value on a recurring basis
Subtotal Assets	1.3	4.2
Subtotal Liabilities	0.3	1.3
Cash surrender value of life insurance | Level 2
Assets and liabilities measured at estimated fair value on a recurring basis
Investments	2.3
Fair value measured on recurring basis
Assets and liabilities measured at estimated fair value on a recurring basis
Investments		2.2
Subtotal Assets	605.9	697.5
Netting and collateral, asset	(356.1)	(380.0)
Total Assets	249.8	317.5
Inventories liability	5.8	22.9
Subtotal Liabilities	578.8	672.5
Netting and collateral, liability	(531.2)	(607.6)
Total, inventories liability	5.8
Total Liabilities	47.6	64.9
Fair value measured on recurring basis | Level 3
Assets and liabilities measured at estimated fair value on a recurring basis
Debt	700.8	689.9
Notes receivable	8.2	7.0
Fair value measured on recurring basis | Cash surrender value of life insurance
Assets and liabilities measured at estimated fair value on a recurring basis
Investments	2.3
Commodity contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total, derivative asset	235.0	305.5
Total, derivative liability	41.8	42.0
Commodity contracts | Level 1
Assets and liabilities measured at estimated fair value on a recurring basis
Derivative asset	207.3	173.7
Derivative liability	339.2	306.4
Commodity contracts | Level 2
Assets and liabilities measured at estimated fair value on a recurring basis
Derivative asset	377.4	495.6
Derivative liability	228.4	329.9
Commodity contracts | Level 3
Assets and liabilities measured at estimated fair value on a recurring basis
Derivative asset	1.3	4.2
Derivative liability	0.3	1.3
Commodity contracts | Fair value measured on recurring basis
Assets and liabilities measured at estimated fair value on a recurring basis
Derivative asset	586.0	673.5
Netting and collateral, derivative asset	(351.0)	(368.0)
Total, derivative asset	235.0	305.5
Derivative liability	567.9	637.6
Netting and collateral, derivative liability	(526.1)	(595.6)
Total, derivative liability	41.8	42.0
Foreign currency contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total, derivative asset	12.5	9.8
Foreign currency contracts | Level 2
Assets and liabilities measured at estimated fair value on a recurring basis
Derivative asset	17.6	21.8
Derivative liability	5.1	12.0
Foreign currency contracts | Fair value measured on recurring basis
Assets and liabilities measured at estimated fair value on a recurring basis
Derivative asset	17.6	21.8
Netting and collateral, derivative asset	(5.1)	(12.0)
Total, derivative asset	12.5	9.8
Derivative liability	5.1	12.0
Netting and collateral, derivative liability	$ (5.1)	$ (12.0)