Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016
Assets:
Cash Equivalents		$ 3,317	$ 6,307
Repurchase Agreements			7,000
Investments:
Total assets		144,849	188,763
Fair Value, Inputs, Level 2 [Member]
Assets:
Cash Equivalents		1,000	4,740
Repurchase Agreements			7,000
Investments:
Total assets		142,532	187,196
Not Subject to Leveling [Member]
Assets:
Cash Equivalents	[1]	2,317	1,567
Investments:
Total assets	[1]	2,317	1,567
Commercial Paper [Member]
Investments:
Investments		16,699	19,689
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		16,699	19,689
Certificates of Deposit [Member]
Investments:
Investments		8,174	10,629
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		8,174	10,629
Corporate Bonds [Member]
Investments:
Investments		74,174	94,609
Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		74,174	94,609
Government Securities [Member]
Investments:
Investments		26,431	33,466
Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		26,431	33,466
Treasury Bonds [Member]
Investments:
Investments		16,054	17,063
Treasury Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		$ 16,054	$ 17,063

[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.