Derivative Instruments - Notional Amounts of Outstanding Derivative Contracts (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) MMBTU	Dec. 31, 2016 USD ($) MMBTU	May 31, 2017 USD ($)
Natural gas contracts
Derivative Instruments
Outstanding derivative contracts that were entered into to hedge forecasted transactions (in mmbtu) | MMBTU	1,280,000	540,000
Foreign exchange contracts
Derivative Instruments
Outstanding derivative contracts that were entered into to hedge forecasted transactions	$ 76,681	$ 199,420
Interest rate swap contract
Derivative Instruments
Outstanding derivative contracts that were entered into to hedge forecasted transactions	$ 50,000		$ 50,000