Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Fair Value Measurements [Line Items]
Available for sale debt securities fair value	$ 43,655,000		$ 43,655,000		$ 51,641,000
Realized gains or losses recognized on the sale or maturity of available-for-sale securities	0	$ 0	0	$ 0
Quoted Prices (Unadjusted) in Active Markets Level 1 [Member]
Fair Value Measurements [Line Items]
Cash and cash equivalents fair value	28,583,000		28,583,000		29,270,000
Quoted Prices (Unadjusted) in Active Markets Level 1 [Member] | Overnight Investments, Money Market Funds and Highly Rated Corporate Bonds Member]
Fair Value Measurements [Line Items]
Cash and cash equivalents fair value	28,600,000		28,600,000		29,300,000
Significant Other Observable Inputs Level 2 [Member]
Fair Value Measurements [Line Items]
Cash and cash equivalents fair value	1,998,000		1,998,000		4,499,000
Significant Other Observable Inputs Level 2 [Member] | Corporate Bonds and Commercial Paper [Member]
Fair Value Measurements [Line Items]
Cash and cash equivalents fair value	2,000,000		2,000,000		4,500,000
Significant Other Observable Inputs Level 2 [Member] | Commercial Paper, Corporate Bonds and Asset Backed Securities [Member] | Short-term Investments [Member]
Fair Value Measurements [Line Items]
Available for sale debt securities fair value	$ 41,700,000		$ 41,700,000		$ 47,100,000