Long-Term Debt - Schedule of Long-Term Debt (Detail) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Debt Issuance Costs, Current, Net $ 3,623 $ 3,671
Long term debt, Principal 1,212,663 1,243,447
Unamortized premium, net [1] 1,227 1,380
Unamortized financing costs (22,138) (26,303)
Current portion [2] (46,324) (44,144)
Long-term debt, less current portion 1,145,428 1,174,380
E1 Arrayan EKF term loan [Member]    
Debt Instrument [Line Items]    
Long term debt, Principal $ 103,904 107,160
Contractual Interest Rate 5.56%  
Effective Interest Rate 5.56%  
Maturity March 2029  
Santa Isabel term loan [Member]    
Debt Instrument [Line Items]    
Long term debt, Principal $ 107,656 109,973
Contractual Interest Rate 4.57%  
Effective Interest Rate 4.57%  
Maturity September 2033  
Ocotillo commercial term loan [Member]    
Debt Instrument [Line Items]    
Long term debt, Principal $ 194,378 208,119
Contractual Interest Rate 2.59%  
Effective Interest Rate [3] 3.79%  
Maturity [4] August 2020  
Lost Creek term loan [Member]    
Debt Instrument [Line Items]    
Long term debt, Principal $ 103,846 110,846
Contractual Interest Rate 2.89%  
Effective Interest Rate [3] 6.50%  
Maturity September 2027  
El Arrayan commercial term loan [Member]    
Debt Instrument [Line Items]    
Long term debt, Principal $ 94,458 97,418
Contractual Interest Rate 3.98%  
Effective Interest Rate [3] 5.69%  
Maturity March 2029  
Spring Valley term loan [Member]    
Debt Instrument [Line Items]    
Long term debt, Principal $ 131,409 132,670
Contractual Interest Rate 2.59%  
Effective Interest Rate [3] 5.31%  
Maturity June 2030  
Ocotillo development term loan [Member]    
Debt Instrument [Line Items]    
Long term debt, Principal $ 103,400 104,500
Contractual Interest Rate 2.94%  
Effective Interest Rate [3] 4.39%  
Maturity August 2033  
St. Joseph term loan [Member]    
Debt Instrument [Line Items]    
Long term debt, Principal $ 166,431 158,181
Contractual Interest Rate 2.52%  
Effective Interest Rate [3] 3.84%  
Maturity [4] November 2033  
Hatchet Ridge financing lease obligation [Member]    
Debt Instrument [Line Items]    
Long term debt, Principal $ 207,181 $ 214,580
Contractual Interest Rate 1.43%  
Effective Interest Rate 1.43%  
Maturity December 2032  
[1] Amount is related to the Lost Creek term loan.
[2] Amount is presented net of the current portion of unamortized financing costs of $3.6 million and $3.7 million as of September 30, 2016 and December 31, 2015, respectively.
[3] Includes impact of interest rate derivatives. Refer to Note 10, Derivative Instruments, for discussion of interest rate derivatives.
[4] The amortization for the Ocotillo commercial term loan and the St. Joseph term loan are through June 2030 and September 2036, respectively, which differs from the stated maturity date of such loans due to prepayment requirements.