Property, Plant and Mine Development (Tables)

6 Months Ended

Jun. 30, 2018

Property, Plant and Equipment [Abstract]

Schedule of Property, Plant and Mine Development

At June 30, 2018 and December 31, 2017, property, plant and mine development (in thousands) consisted of the following:

	June 30,   2018			December 31,    2017
Mining property and mine development	$	1,051,824		$	586,242
Asset retirement cost	14,308			14,184
Land	42,189			36,552
Land improvements	66,336			45,878
Buildings	73,150			56,330
Machinery and equipment	790,655			590,566
Furniture and fixtures	4,053			2,953
Construction-in-progress	183,222			189,970
	2,225,737			1,522,675
Accumulated depletion, depreciation and amortization	(400,084		)	(353,520		)
Total property, plant and mine development, net	$	1,825,653		$	1,169,155