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Schedule II - Valuation and Qualifying Accounts (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounts Receivable - Allowance for Doubtful Accounts [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Year $ 357 $ 525 $ 684
Additions- Charges to (Recovery of) Costs and Expenses (54) 80 224
Deductions- Write- offs/Costs Incurred 0 248 383
Balance at End of Year 303 357 525
Supplies-Reserve for Slow-Moving Items [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Year 15 928 928
Additions- Charges to (Recovery of) Costs and Expenses 0 0 0
Deductions- Write- offs/Costs Incurred 0 913 0
Balance at End of Year 15 15 928
Notes Receivable - Allowance for Doubtful Accounts [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Year 0 970 970
Additions- Charges to (Recovery of) Costs and Expenses 0 0 0
Deductions- Write- offs/Costs Incurred 0 970 0
Balance at End of Year 0 0 970
Deferred Tax Assets - Valuation Allowance [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Year 13,128 1,242 292
Additions- Charges to (Recovery of) Costs and Expenses 13,792 11,886 950
Deductions- Write- offs/Costs Incurred 0 0 0
Balance at End of Year $ 26,920 $ 13,128 $ 1,242