Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 1,095,990	$ 755,103
Less accumulated amortization and depreciation	428,249	345,085
Net property, plant and equipment	667,741	410,018
Plant in Service [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	1,020,014	718,503
Plant under Construction [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 75,976	$ 36,600