Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 714,588	$ 693,399
Less accumulated amortization and depreciation	311,711	287,492
Net property, plant and equipment	402,877	405,907
Plant in Service [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	692,038	675,321
Plant under Construction [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 22,550	$ 18,078