Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Assets:
Cash Equivalents		$ 39,529	$ 25,964
Investments:
Total assets		56,706	139,859
Fair Value, Inputs, Level 2 [Member]
Assets:
Cash Equivalents		500
Investments:
Total assets		17,677	113,895
Not Subject to Leveling [Member]
Assets:
Cash Equivalents	[1]	39,029	25,964
Investments:
Total assets	[1]	39,029	25,964
Commercial Paper [Member]
Investments:
Investments		1,397	6,198
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		1,397	6,198
Corporate Bonds [Member]
Investments:
Investments		4,296	58,865
Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		4,296	58,865
Treasury Bonds [Member]
Investments:
Investments		11,484	16,962
Treasury Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		$ 11,484	16,962
Certificates of Deposit [Member]
Investments:
Investments			8,916
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments			8,916
Government Securities [Member]
Investments:
Investments			22,954
Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments			$ 22,954

[1] Certain cash equivalents and investments that are valued using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy