Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 1,040,550	$ 755,103
Less accumulated amortization and depreciation	387,027	345,085
Net property, plant and equipment	653,523	410,018
Plant in Service [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	964,601	718,503
Plant under Construction [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 75,949	$ 36,600