Derivative Instruments (Details 3) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 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	925,000	810,000
Foreign exchange contracts
Derivative Instruments
Outstanding derivative contracts that were entered into to hedge forecasted transactions	$ 9,600	$ 41,237