Significant Accounting Policies - Schedule of Amounts Recorded in Prepaid Expenses and Other (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Prepaid Expense And Other Assets Current [Abstract]
Derivative assets - current	[1]		$ 10,036
Prepaid/accrued insurance and surety bonds		$ 5,386	7,475
Prepaid deposits related to royalties		6,237	5,943
Other	[2]	2,881	3,425
Prepaid expenses and other		$ 14,504	$ 26,879

[1] Includes open and closed (and not yet collected) derivative commodity contracts recorded at fair value as of December 31, 2015.

[2] Individual items were less than 5% of total current assets for either period presented.