Components of Working Capital - Accrued Liabilities Table (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Components Of Working Capital [Abstract]
Accrued interest expense		$ 40,646	$ 39,834
Short-term asset retirement obligations		2,304	3,114
Short-term remediation and reclamation obligations		833	630
Income taxes payable		2,495	1,006
Other accrued liabilities		1,714	606
Total accrued liabilities	[1]	$ 47,992	$ 45,190

[1] Accrued liabilities includes affiliate amounts of $0.2 million and zero as of December 31, 2017 and 2016, respectively.