Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 667,722	$ 656,661
Less accumulated amortization and depreciation	261,993	247,698
Property, plant and equipment, net	405,729	408,963
Plant in Service [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	654,586	633,480
Plant under Construction [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 13,136	$ 23,181

The cumulative amount of depreciation, depletion and amortization (related to property, plant and equipment, but not including land) that has been recognized in the income statement.

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Gross amount of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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- Definition

Amount, net of accumulated depreciation, depletion and amortization, of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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