Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Prepayments and other	$ 5,912	$ 6,918
Fair Value Measured on a Recurring Basis | Fair Value Total
Derivatives, Fair Value [Line Items]
Prepayments and other		6,900
Not Designated as Hedging Instrument | Fair Value Measured on a Recurring Basis | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		52
Not Designated as Hedging Instrument | Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0
Not Designated as Hedging Instrument | Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		52
Not Designated as Hedging Instrument | Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0
Not Designated as Hedging Instrument | Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		52
Not Designated as Hedging Instrument | Fair Value Measured on a Recurring Basis | Netting | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		$ 0