Fair Value Measurements - Carrying Value and Fair Value of Certain Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 32,838	$ 111,897
Marketable securities - held-to-maturity	40,763	44,628
Carrying Value [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities - held-to-maturity		6,596
Carrying Value [Member] | U.S. Treasury Bills [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities - held-to-maturity		1,503
Carrying Value [Member] | US Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities - held-to-maturity	1,329	503
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	39,434	36,026
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	32,838	111,897
Marketable securities - held-to-maturity	40,763	44,628
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
Marketable securities - held-to-maturity	0	0
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
Marketable securities - held-to-maturity	0	0
Estimate of Fair Value Measurement [Member] | Commercial Paper [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities - held-to-maturity		6,596
Estimate of Fair Value Measurement [Member] | Commercial Paper [Member] | Level 2 [Member]
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] | Commercial Paper [Member] | Level 3 [Member]
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] | U.S. Treasury Bills [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities - held-to-maturity		1,503
Estimate of Fair Value Measurement [Member] | U.S. Treasury Bills [Member] | Level 2 [Member]
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] | U.S. Treasury Bills [Member] | Level 3 [Member]
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] | US Government Agencies Debt Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities - held-to-maturity	1,329	503
Estimate of Fair Value Measurement [Member] | US Government Agencies Debt Securities [Member] | Level 2 [Member]
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] | US Government Agencies Debt Securities [Member] | Level 3 [Member]
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] | 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	39,434	36,026
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
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