Revenue (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
Disaggregation of Revenue [Line Items]
Total Revenue from Contracts with Customers									$ 1,472,790	$ 1,321,412	$ 1,143,514
Coal Revenue
Disaggregation of Revenue [Line Items]
Total Revenue from Contracts 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
Terminal Revenue
Disaggregation of Revenue [Line Items]
Total Revenue from Contracts with Customers	16,931	16,115	16,659	15,221	17,260	15,065	14,855	12,886	64,926	60,066	31,464
Freight Revenue
Disaggregation of Revenue [Line Items]
Total Revenue from Contracts with Customers	$ 5,798	$ 2,443	$ 17,444	$ 17,887	$ 21,845	$ 21,803	$ 17,762	$ 12,282	$ 43,572	$ 73,692	$ 46,468