Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 1,186,426	$ 1,159,077
Less accumulated amortization and depreciation	496,478	460,955
Net property, plant and equipment	689,948	698,122
Plant in service
Property, plant and equipment [Abstract]
Total property, plant and equipment	1,124,446	1,085,318
Plant under construction
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 61,980	$ 73,759