Derivative Instruments - Notional Amounts of Outstanding Derivative Contracts (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) MMBTU	Dec. 31, 2015 USD ($) MMBTU	Dec. 31, 2014 USD ($) MMBTU
Natural gas contracts
Derivative Instruments
Outstanding derivative contracts that were entered into to hedge forecasted transactions (in mmbtu) | MMBTU	540,000	955,000	810,000
Foreign exchange contracts
Derivative Instruments
Outstanding derivative contracts that were entered into to hedge forecasted transactions | $	$ 199,420	$ 37,016	$ 41,237