Segment Information (Schedule of Industry Segment Results) (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
Segment Reporting Information [Line Items]
Coal Revenue	$ 288,254	$ 279,245	$ 303,707	$ 316,448	$ 321,171	$ 267,685	$ 250,562	$ 226,164	$ 1,187,654	$ 1,065,582	$ 1,289,036
Terminal Revenue									60,066	31,464	30,967
Freight Revenue	21,845	21,803	17,762	12,282	12,519	9,392	11,447	13,110	73,692	46,468	20,499
Total Revenue and Freight									1,321,412	1,143,514	1,340,502
Earnings (Loss) Before Income Tax	39,801	12,288	61,857	55,851	37,738	4,685	13,792	8,800	169,797	65,015	443,026
Segment Assets	2,707,099				2,687,434				2,707,099	2,687,434	2,867,733
Depreciation, Depletion and Amortization	47,088	$ 46,653	$ 25,268	$ 52,993	50,296	$ 49,850	$ 29,314	$ 48,662	172,002	178,122	195,337
Capital Expenditures									81,413	53,600	143,053
Customer Concentration Risk | Sales Revenue, Goods, Net | Duke Energy
Segment Reporting Information [Line Items]
Coal Revenue									222,354	160,818	242,020
Customer Concentration Risk | Sales Revenue, Goods, Net | Xcoal Energy Resources
Segment Reporting Information [Line Items]
Coal Revenue									145,248		150,199
Customer Concentration Risk | Sales Revenue, Goods, Net | GenOn Energy Management, LLC
Segment Reporting Information [Line Items]
Coal Revenue										116,849	157,174
PAMC
Segment Reporting Information [Line Items]
Coal Revenue									1,187,654	1,065,582	1,289,036
Terminal Revenue									0	0	0
Freight Revenue									73,692	46,468	20,499
Total Revenue and Freight									1,261,346	1,112,050	1,309,535
Earnings (Loss) Before Income Tax									189,162	130,708	404,994
Segment Assets	1,971,268				1,982,206				1,971,268	1,982,206	2,076,301
Depreciation, Depletion and Amortization									166,628	168,195	176,864
Capital Expenditures									77,981	50,809	136,291
Other
Segment Reporting Information [Line Items]
Coal Revenue									0	0	0
Terminal Revenue									60,066	31,464	30,967
Freight Revenue									0	0	0
Total Revenue and Freight									60,066	31,464	30,967
Earnings (Loss) Before Income Tax									(19,365)	(65,693)	38,032
Segment Assets	$ 735,831				$ 705,228				735,831	705,228	791,432
Depreciation, Depletion and Amortization									5,374	9,927	18,473
Capital Expenditures									$ 3,432	$ 2,791	$ 6,762