Conveyance of Future Hard Minerals Royalty Revenues - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Mar. 31, 2016
Deferred Revenue Disclosure [Abstract]
Proceeds from conveyance of future hard minerals royalty revenues, net of transaction costs	$ 413	$ 0
Maximum amount of total future royalty revenue third-party is entitled to	528		$ 553
Deferred Revenue Arrangement [Line Items]
Amount of deferred revenue amortized in period	27
Payments of future hard minerals royalty revenues conveyed	$ 25	$ 0
Royalties Between $800 and $900 Million During the First 10 Years [Member]
Deferred Revenue Arrangement [Line Items]
Percentage of excess royalties	3.00%
Baseline amount of excess royalties	$ 800
Royalties In Excess Of $900 Million During the First 10 Years [Member]
Deferred Revenue Arrangement [Line Items]
Percentage of excess royalties	4.00%
Baseline amount of excess royalties	$ 900
Minimum [Member]
Deferred Revenue Arrangement [Line Items]
Term of conveyance	10 years
Maximum [Member]
Deferred Revenue Arrangement [Line Items]
Term of conveyance	15 years