Income Taxes Schedule of Reconciliation of Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amount
Statutory U.S. federal income tax rate									$ 59,429	$ 22,755	$ 155,059
State income taxes, net of federal tax benefit									1,264	997	6,767
Excess tax depletion									(24,216)	(21,856)	(27,720)
Effect of domestic production activities									(6,493)	1,621	(4,933)
Effect of change in U.S. tax law									58,558	0	0
IRS and state tax examination settlements									0	13,958	0
Effect of valuation allowance									1,379	0	0
Other									(2,693)	(2,910)	(3,568)
Income Tax Expense / Effective Rate	$ 64,441	$ 3,770	$ 9,611	$ 9,406	$ 14,824	$ (66)	$ (109)	$ (84)	$ 87,228	$ 14,565	$ 125,605
Percent
Statutory U.S. federal income tax rate (as a percent)									35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit (as a percent)									0.70%	1.50%	1.50%
Excess tax depletion (as a percent)									(14.30%)	(33.60%)	(6.30%)
Effect of domestic production activities (as a percent)									(3.80%)	2.50%	(1.10%)
Effect of change in U.S. tax law (as a percent)									34.50%	0.00%	0.00%
IRS and state tax examination settlements (as a percent)									0.00%	21.50%	0.00%
Effect of valuation allowance (as a percent)									0.80%	0.00%	0.00%
Other (as a percent)									(1.60%)	(4.50%)	(0.80%)
Income Tax Expense / Effective Rate (as a percent)									51.30%	22.40%	28.30%