Schedule of Intangible Assets, Including Impact of Impairment Charges (Detail) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Finite Lived Intangible Assets [Line Items]
Intangible Assets, Accumulated Amortization	$ (69,269)	$ (66,065)
Goodwill, Cost	205,211	198,370
Goodwill, Net	205,211	198,370
Total Goodwill and Intangible Assets, Cost	308,611	301,570
Total Goodwill and Intangible Assets, Net	$ 239,342	$ 235,505
Customer Contracts And Relationships [Member]
Finite Lived Intangible Assets [Line Items]
Intangible Assets, Useful Life	15 years	15 years
Intangible Assets, Cost	$ 72,260	$ 72,260
Intangible Assets, Accumulated Amortization	(58,732)	(56,303)
Intangible Assets, Net	$ 13,528	$ 15,957
Sales Contracts [Member]
Finite Lived Intangible Assets [Line Items]
Intangible Assets, Useful Life	4 years	4 years
Intangible Assets, Cost	$ 2,500	$ 2,500
Intangible Assets, Accumulated Amortization	$ (2,500)	$ (2,500)
Permits [Member]
Finite Lived Intangible Assets [Line Items]
Intangible Assets, Useful Life	40 years	40 years
Intangible Assets, Cost	$ 28,640	$ 28,440
Intangible Assets, Accumulated Amortization	(8,037)	(7,262)
Intangible Assets, Net	$ 20,603	$ 21,178