Fair Value Measurements - Commodity and Foreign Currency Contracts (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Offsetting Assets [Line Items]
Gross Amounts Recognized	$ 559.1	$ 312.0
Gross Amounts Offset	475.4	235.2
Net Amounts Presented	83.7	76.9
Cash Collateral	0.0	21.2
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	83.7	55.7
Offsetting Liabilities [Line Items]
Gross Amounts Recognized	581.6	332.5
Gross Amounts Offset	475.4	235.2
Net Amounts Presented	106.2	97.4
Cash Collateral	0.0	39.2
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	106.2	58.2
Commodity contracts
Offsetting Assets [Line Items]
Gross Amounts Recognized	556.7	303.6
Gross Amounts Offset	474.2	228.4
Net Amounts Presented	82.5	75.1
Cash Collateral	0.0	21.2
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	82.5	53.9
Offsetting Liabilities [Line Items]
Gross Amounts Recognized	579.0	323.9
Gross Amounts Offset	474.2	228.4
Net Amounts Presented	104.8	95.4
Cash Collateral	0.0	39.2
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	104.8	56.2
Foreign currency contracts
Offsetting Assets [Line Items]
Gross Amounts Recognized	2.4	8.5
Gross Amounts Offset	1.2	6.7
Net Amounts Presented	1.2	1.7
Cash Collateral	0.0	0.0
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	1.2	1.7
Offsetting Liabilities [Line Items]
Gross Amounts Recognized	2.6	8.7
Gross Amounts Offset	1.2	6.7
Net Amounts Presented	1.4	2.0
Cash Collateral	0.0	0.0
Gross Amounts without Right of Offset	0.0	0.0
Net Amounts	$ 1.4	$ 2.0