Fair Value Measurements - Carrying Value and Fair Value of Certain Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term restricted cash equivalents	$ 0	$ 6,336
Trading securities	1,307	3,761
Reported Value Measurement [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	44,522	20,751
Long-term restricted cash equivalents		6,336
Trading securities	1,307	3,761
Marketable securities—held-to-maturity:	25,114
Reported Value Measurement [Member] | Fair Value, Measurements, Recurring | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	249	4,452
Reported Value Measurement [Member] | Fair Value, Measurements, Recurring | Government agency bond
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	8,471
Reported Value Measurement [Member] | Fair Value, Measurements, Recurring | U.S. Treasury Bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	16,394
Reported Value Measurement [Member] | Fair Value, Measurements, Recurring | Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit facility repayment feature liability	5,622
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	44,522	20,751
Long-term restricted cash equivalents		6,336
Trading securities	1,307	3,761
Marketable securities—held-to-maturity:	24,865
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Level 2
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
Trading securities	0	0
Marketable securities—held-to-maturity:	249
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Level 3
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
Trading securities	0	0
Marketable securities—held-to-maturity:	0
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Certificates of deposit | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	0	4,452
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Certificates of deposit | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	249	0
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Certificates of deposit | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	0	$ 0
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Government agency bond | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	8,471
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Government agency bond | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	0
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Government agency bond | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	0
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | U.S. Treasury Bills | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	16,394
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | U.S. Treasury Bills | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	0
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | U.S. Treasury Bills | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities—held-to-maturity:	0
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Debt | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit facility repayment feature liability	0
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Debt | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit facility repayment feature liability	0
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring | Debt | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit facility repayment feature liability	$ 5,622