Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 1,254,898	$ 1,159,077
Less accumulated amortization and depreciation	571,543	460,955
Net property, plant and equipment	683,355	698,122
Plant in service
Property, plant and equipment [Abstract]
Total property, plant and equipment	1,187,799	1,085,318
Plant under construction
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 67,099	$ 73,759