Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 1,302,084	$ 1,281,387
Less accumulated amortization and depreciation	630,067	595,060
Net property, plant and equipment	672,017	686,327
Plant in service
Property, plant and equipment [Abstract]
Total property, plant and equipment	1,245,079	1,219,185
Plant under construction
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 57,005	$ 62,202