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Fair Value Measurements - Carrying Value and Fair Value of Certain Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term restricted cash equivalents $ 6,336,000 $ 6,336,000
Trading securities 3,761,000 0
Fair Value, Measurements, Recurring [Member] | Carrying Value [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 20,751,000 22,058,000
Long-term restricted cash equivalents 6,336,000 6,336,000
Trading securities 3,761  
Fair Value, Measurements, Recurring [Member] | Carrying Value [Member] | Certificates of deposit [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities - held-to-maturity 4,452,000 15,686,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 20,751,000 22,058,000
Long-term restricted cash equivalents 6,336,000 6,336,000
Trading securities 3,761  
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0 0
Long-term restricted cash equivalents 0 0
Trading securities 0  
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0 0
Long-term restricted cash equivalents 0 0
Trading securities 0  
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Certificates of deposit [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities - held-to-maturity 4,452,000 15,686,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Certificates of deposit [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities - held-to-maturity 0 0
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Certificates of deposit [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities - held-to-maturity $ 0 $ 0