Segment Information (Schedule of Industry Segment Results) (Details) - USD ($) $ 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, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenue from Contract with Customers									$ 1,472,790	$ 1,321,412	$ 1,143,514
Earnings (Loss) Before Income Tax	$ 46,335	$ 8,394	$ 55,741	$ 77,143	$ 39,801	$ 12,288	$ 61,857	$ 55,851	187,613	169,797	65,015
Segment Assets	2,760,727				2,707,099				2,760,727	2,707,099	2,687,434
Depreciation, Depletion and Amortization	45,590	51,242	54,961	49,471	47,088	46,653	25,268	52,993	201,264	172,002	178,122
Capital Expenditures									145,749	81,413	53,600
PAMC
Segment Reporting Information [Line Items]
Revenue from Contract with Customers									1,407,864	1,261,346	1,112,050
Earnings (Loss) Before Income Tax									291,418	189,162	130,708
Segment Assets	1,894,209				1,971,268				1,894,209	1,971,268	1,982,206
Depreciation, Depletion and Amortization									178,969	166,628	168,195
Capital Expenditures									124,570	77,981	50,809
Other
Segment Reporting Information [Line Items]
Revenue from Contract with Customers									64,926	60,066	31,464
Earnings (Loss) Before Income Tax									(103,805)	(19,365)	(65,693)
Segment Assets	866,518				735,831				866,518	735,831	705,228
Depreciation, Depletion and Amortization									22,295	5,374	9,927
Capital Expenditures									21,179	3,432	2,791
Coal Revenue
Segment Reporting Information [Line Items]
Revenue from Contract with Customers	347,789	294,797	370,697	351,009	288,254	279,245	303,707	316,448	1,364,292	1,187,654	1,065,582
Coal Revenue | PAMC
Segment Reporting Information [Line Items]
Revenue from Contract with Customers									1,364,292	1,187,654	1,065,582
Coal Revenue | Other
Segment Reporting Information [Line Items]
Revenue from Contract with Customers									0	0	0
Terminal Revenue
Segment Reporting Information [Line Items]
Revenue from Contract with Customers	16,931	16,115	16,659	15,221	17,260	15,065	14,855	12,886	64,926	60,066	31,464
Terminal Revenue | PAMC
Segment Reporting Information [Line Items]
Revenue from Contract with Customers									0	0	0
Terminal Revenue | Other
Segment Reporting Information [Line Items]
Revenue from Contract with Customers									64,926	60,066	31,464
Freight Revenue
Segment Reporting Information [Line Items]
Revenue from Contract with Customers	$ 5,798	$ 2,443	$ 17,444	$ 17,887	$ 21,845	$ 21,803	$ 17,762	$ 12,282	43,572	73,692	46,468
Freight Revenue | PAMC
Segment Reporting Information [Line Items]
Revenue from Contract with Customers									43,572	73,692	46,468
Freight Revenue | Other
Segment Reporting Information [Line Items]
Revenue from Contract with Customers									$ 0	$ 0	$ 0