Income Taxes (Schedule of Reconciliation of Income Tax Expense) (Benefit) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Amount
Statutory U.S. federal income tax rate									$ 20,600	$ 39,399	$ 59,429
State income taxes, net of federal tax benefit									3,125	3,240	1,264
Foreign income taxes									1,336	1,436	0
Excess tax depletion									(13,141)	(20,873)	(24,216)
Effect of domestic production activities									0	0	(6,493)
Effect of change in U.S. tax law									0	2,777	58,558
Excess compensation									1,849	974	0
Effect of valuation allowance									1,400	(1,379)	1,379
Tax credits									(2,536)	(980)	0
Non-controlling interest									(3,687)	(5,420)	0
State rate change and prior period adjustments									(5,745)	(8,223)	0
Other									1,338	(2,123)	(2,693)
Income Tax Expense / Effective Rate	$ 4,782	$ 2,415	$ (1,808)	$ (850)	$ 301	$ (690)	$ 3,032	$ 6,185	$ 4,539	$ 8,828	$ 87,228
Percent
Statutory U.S. federal income tax rate (as a percent)									21.00%	21.00%	35.00%
State income taxes, net of federal tax benefit (as a percent)									3.20%	1.70%	0.70%
Foreign income taxes (as a percent)									1.40%	0.80%	0.00%
Excess tax depletion (as a percent)									(13.40%)	(11.10%)	(14.30%)
Effect of domestic production activities (as a percent)									0.00%	0.00%	(3.80%)
Effect of change in U.S. tax law (as a percent)									0.00%	1.50%	34.50%
Excess compensation (as a percent)									1.90%	0.50%	0.00%
Effect of valuation allowance (as a percent)									1.40%	(0.70%)	0.80%
Tax credits (as a percent)									(2.60%)	(0.50%)	0.00%
Non-controlling interest (as a percent)									(3.80%)	(2.90%)	0.00%
State rate change and prior period adjustments (as a percent)									(5.90%)	(4.40%)	0.00%
Other (as a percent)									1.40%	(1.10%)	(1.60%)
Income Tax Expense / Effective Rate (as a percent)									4.60%	4.80%	51.30%