Derivative Instruments - Notional Amounts (Details) therm in Thousands, $ in Thousands	Jun. 30, 2017 USD ($) therm	Dec. 31, 2016 USD ($) therm	Jun. 30, 2016 USD ($) therm
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Natural gas - financial (in therms)	490,780	477,430	517,980
Natural gas - physical (in therms)	495,751	535,450	398,980
Foreign exchange \| $	$ 7,788	$ 7,497	$ 7,254

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant