Derivatives - Gross Notional Values (Details) bbl in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) bbl
Long | Commodity contracts
Derivative instruments, at their respective fair value positions
Notional value of commodity contracts (in barrels) | bbl	77.5
Long | Foreign currency contracts | USD
Derivative instruments, at their respective fair value positions
Notional amount of foreign currency exchange contracts | $	$ 401.2
Short | Commodity contracts
Derivative instruments, at their respective fair value positions
Notional value of commodity contracts (in barrels) | bbl	87.7
Short | Foreign currency contracts | USD
Derivative instruments, at their respective fair value positions
Notional amount of foreign currency exchange contracts | $	$ 275.6