Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Detail) - Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Quoted Prices in Active Markets (Level 1)
Assets -
Marketable securities	$ 7,321	$ 6,246
FCX Settlement		0
Significant Other Observable Inputs (Level 2)
Assets -
Marketable securities	0	0
FCX Settlement		0
Significant Unobservable Inputs (Level 3)
Assets -
Marketable securities	0	0
FCX Settlement		14,400
Carrying Value
Assets -
Marketable securities	$ 7,321	6,246
FCX Settlement		$ 14,400