Conveyance of Future Hard Minerals Royalty Revenues - Schedule of Future Royalty Payments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Mar. 31, 2016
Deferred Revenue Disclosure [Abstract]
2018	$ 50
2019	52
2020	56
2021	57
2022	58
Later years	204
Total	$ 477	$ 553

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