Segment Information - Revenue from External Customers (Details) T in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) T	Sep. 30, 2017 USD ($) T	Sep. 30, 2018 USD ($) T	Sep. 30, 2017 USD ($) T
Revenues from External Customers and Long-Lived Assets [Line Items]
Coal purchased from third parties excluding coal sold under Back-to-Back Coal Supply Agreements | T	1,705	1,320	4,938	3,727
Product Supplied By Third Party Concentration Risk | Coal Purchased From Third Parties Sold
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk	44.00%	35.00%	41.00%	31.00%
Non-US | Geographic Concentration Risk | Revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk	87.00%	77.00%	88.00%	76.00%
INDIA
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from contract with customer	$ 62,929	$ 68,206	$ 352,344	$ 268,924
TURKEY
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from contract with customer	58,402
BRAZIL
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from contract with customer	56,519	36,867	218,486
ITALY
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from contract with customer				125,464
Coal
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from contract with customer	443,005	380,670	1,446,538	1,291,489
Coal | Non-US
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from contract with customer	$ 384,597	$ 291,627	$ 1,267,642	$ 984,965