Derivatives - Gross Notional Values (Details) bbl in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) bbl
Long | Commodity contracts
Derivative instruments, at their respective fair value positions
Notional value of commodity contracts (in barrels) | bbl	77.2
Long | Foreign currency contracts | USD
Derivative instruments, at their respective fair value positions
Notional amount of foreign currency exchange contracts | $	$ 188.4
Short | Commodity contracts
Derivative instruments, at their respective fair value positions
Notional value of commodity contracts (in barrels) | bbl	74.3
Short | Foreign currency contracts | USD
Derivative instruments, at their respective fair value positions
Notional amount of foreign currency exchange contracts | $	$ 96.9