Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 4,627,864	$ 4,747,351
Less: accumulated depreciation	(830,766)	(782,230)
Property, plant and equipment, net	3,797,098	3,965,121
Operating Wind Farms [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,521,482	4,640,718
Transmission Line [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	93,849	93,849
Furniture, Fixtures and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	12,285	12,643
Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 248	$ 141