Fair Value Measurements - Additional Information (Detail) - USD ($)	36 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Measurements [Line Items]
Available for sale debt securities fair value	$ 51,641,000	$ 108,248,000
Realized gains or losses recognized on the sale or maturity of available-for-sale securities	0
Quoted Prices in Active Markets Level 1 [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	29,270,000	24,972,000
Quoted Prices in Active Markets Level 1 [Member] | Overnight Investments, Money Market Funds and Highly Rated Corporate Bonds Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	29,300,000	25,000,000
Significant Other Observable Inputs Level 2 [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	4,499,000	10,196,000
Significant Other Observable Inputs Level 2 [Member] | Corporate Bonds and Commercial Paper [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	4,500,000	10,200,000
Significant Other Observable Inputs Level 2 [Member] | Corporate Bonds and Commercial Paper [Member] | Short-term Investments [Member]
Fair Value Measurements [Line Items]
Available for sale debt securities fair value	$ 47,100,000	94,800,000
Significant Other Observable Inputs Level 2 [Member] | Corporate Bonds and Commercial Paper [Member] | Long-term Investments [Member]
Fair Value Measurements [Line Items]
Available for sale debt securities fair value		$ 3,200,000