Fair Value Measurements - Commodity and Foreign Currency Contracts (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Offsetting Assets [Line Items]
Gross Amounts Recognized	$ 533.9	$ 312.0
Gross Amounts Offset	439.2	235.2
Net Amounts Presented	94.7	76.9
Cash Collateral	0.0	21.2
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	94.7	55.7
Offsetting Liabilities [Line Items]
Gross Amounts Recognized	549.2	332.5
Gross Amounts Offset	439.2	235.2
Net Amounts Presented	110.0	97.4
Cash Collateral	0.0	39.2
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	110.0	58.2
Commodity contracts
Offsetting Assets [Line Items]
Gross Amounts Recognized	515.0	303.6
Gross Amounts Offset	431.9	228.4
Net Amounts Presented	83.2	75.1
Cash Collateral	0.0	21.2
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	83.2	53.9
Offsetting Liabilities [Line Items]
Gross Amounts Recognized	540.4	323.9
Gross Amounts Offset	431.9	228.4
Net Amounts Presented	108.5	95.4
Cash Collateral	0.0	39.2
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	108.5	56.2
Foreign currency contracts
Offsetting Assets [Line Items]
Gross Amounts Recognized	18.9	8.5
Gross Amounts Offset	7.3	6.7
Net Amounts Presented	11.6	1.7
Cash Collateral	0.0	0.0
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	11.6	1.7
Offsetting Liabilities [Line Items]
Gross Amounts Recognized	8.8	8.7
Gross Amounts Offset	7.3	6.7
Net Amounts Presented	1.5	2.0
Cash Collateral	0.0	0.0
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	$ 1.5	$ 2.0