Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 4,357,693	$ 3,717,271
Less: accumulated depreciation	(675,424)	(582,109)
Property, plant and equipment, net	3,682,269	3,135,162
Operating wind farms [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,346,605	3,707,823
Furniture, fixtures and equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	10,947	9,307
Transmission line [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		0
Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 141	$ 141