Derivative Instruments - Notional Amounts of Outstanding Derivative Contracts (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) MMBTU	Dec. 31, 2015 USD ($) MMBTU
Natural gas contracts
Derivative Instruments
Outstanding derivative contracts that were entered into to hedge forecasted transactions (in mmbtu) | MMBTU	720,000	955,000
Foreign exchange contracts
Derivative Instruments
Outstanding derivative contracts that were entered into to hedge forecasted transactions | $	$ 42,697	$ 37,016