CONDENSED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues:
Total revenues	$ 273,304,000	$ 311,955,000	$ 1,017,647,000	$ 929,289,000
Costs and expenses:
Cost of sales (exclusive of items shown separately below)	167,188,000	189,564,000	536,407,000	455,860,000
Cost of other revenues (exclusive of items shown separately below)	6,704,000	6,985,000	21,826,000	22,959,000
Depreciation and depletion	26,071,000	23,393,000	71,750,000	57,625,000
Selling, general and administrative	7,357,000	9,243,000	29,056,000	23,073,000
Transaction and other expenses	3,265,000	0	7,539,000	12,873,000
Total costs and expenses	210,585,000	229,185,000	666,578,000	572,390,000
Operating income	62,719,000	82,770,000	351,069,000	356,899,000
Interest expense, net	(10,128,000)	(640,000)	(28,472,000)	(1,890,000)
Income before income tax benefit	52,591,000	82,130,000	322,597,000	355,009,000
Income tax benefit	0	(37,587,000)	0	(2,881,000)
Net income	$ 52,591,000	$ 119,717,000	$ 322,597,000	$ 357,890,000
Basic and diluted net income per share:
Net income per share—basic (in dollars per share)	$ 1.00	$ 2.27	$ 6.10	$ 6.79
Net income per share—diluted (in dollars per share)	$ 1.00	$ 2.27	$ 6.09	$ 6.79
Weighted average number of shares outstanding—basic (in shares)	52,707	52,777	52,916	52,727
Weighted average number of shares outstanding—diluted (in shares)	52,708	52,777	52,945	52,727
Dividends per share (in dollars per share)	$ 0.05	$ 0.05	$ 6.68	$ 3.66
Sales
Revenues:
Total revenues	$ 264,908,000	$ 302,958,000	$ 992,832,000
Other revenues
Revenues:
Total revenues	$ 8,396,000	$ 8,997,000	$ 24,815,000	$ 33,487,000