Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 728,563	$ 693,399
Less accumulated amortization and depreciation	330,128	287,492
Net property, plant and equipment	398,435	405,907
Plant in Service [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	700,828	675,321
Plant under Construction [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 27,735	$ 18,078