Quarterly Selected Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 650,963	$ 633,180	$ 592,349	$ 575,295	$ 560,244	$ 613,538	$ 549,866	$ 600,975	$ 575,688	$ 2,451,787	$ 2,324,623
Gross profit	101,396	109,740	77,663	81,198	63,460	66,145	63,150	86,707
Asset impairment and mine closure costs									0	0	0
Income from operations	86,490	82,886	44,595	65,167	51,117	73,310	42,924	66,985	46,086	279,138	234,336
Reorganization items, net	(60)	(560)	(740)	(301)	494	(43)	(21)	(2,828)	(759)	(1,661)	(2,398)
Net income	$ 86,094	$ 123,192	$ 43,306	$ 59,985	$ 81,271	$ 68,351	$ 37,160	$ 51,668	$ 33,449	$ 312,577	$ 238,450
Diluted earnings per common share (in dollars per share)	$ 4.44	$ 6.10	$ 2.06	$ 2.74	$ 3.64	$ 2.83	$ 1.48	$ 2.03	$ 1.31	$ 15.15	$ 9.84