Conveyance of Future Hard Minerals Royalty Revenues - Schedule of Future Royalty Payments (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Mar. 31, 2016
Deferred Revenue Disclosure [Abstract]
2017	$ 50
2018	50
2019	52
2020	56
2021	57
Later years	263
Total	$ 528	$ 553

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