Business Segments - Operating Results for Two Business Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 13,498	$ 14,933	$ 11,049	$ 8,923	$ 16,195	$ 27,148	$ 18,507	$ 13,345	$ 48,403	$ 75,195	$ 96,023
Operating loss	$ (5,808)	$ (3,851)	$ (7,805)	$ (14,693)	$ (6,988)	$ (4,060)	$ (7,150)	$ (9,344)	(32,157)	(27,542)	(20,118)
Unallocated corporate expenses
Segment Reporting Information [Line Items]
Operating loss									(4,290)	(2,892)	(4,027)
Wind
Segment Reporting Information [Line Items]
Revenues									14,294	47,269	68,883
Wind | Operating Segments
Segment Reporting Information [Line Items]
Operating loss									(8,904)	(4,174)	(1,256)
Grid
Segment Reporting Information [Line Items]
Revenues									34,109	27,926	27,140
Grid | Operating Segments
Segment Reporting Information [Line Items]
Operating loss									$ (18,963)	$ (20,476)	$ (14,835)