Provision for Site Reclamation and Closure (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Provision For Site Reclamation And Closure [Abstract]

Summary of Detailed Information about Provision for Site Reclamation and Closure

Balance at December 31, 2015	$	3,981
Accretion of discounted cash flows		56
Change in estimated cash flows and assumptions		(889	)
Balance at December 31, 2016	$	3,148
Accretion of discounted cash flows		230
Change in estimated cash flows and assumptions		723
Balance at December 31, 2017	$	4,101