Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 682,948	$ 656,661
Less accumulated amortization and depreciation	277,138	247,698
Net property, plant and equipment	405,810	408,963
Plant in Service [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	666,830	633,480
Plant under Construction [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 16,118	$ 23,181