Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016
Assets:
Cash Equivalents		$ 5,110	$ 6,307
Repurchase Agreements		4,000	7,000
Investments:
Total assets		169,383	188,763
Fair Value, Inputs, Level 2 [Member]
Assets:
Cash Equivalents			4,740
Repurchase Agreements		4,000	7,000
Investments:
Total assets		164,273	187,196
Not Subject to Leveling [Member]
Assets:
Cash Equivalents	[1]	5,110	1,567
Investments:
Total assets	[1]	5,110	1,567
Commercial Paper [Member]
Investments:
Investments		24,182	19,689
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		24,182	19,689
Certificates of Deposit [Member]
Investments:
Investments		10,382	10,629
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		10,382	10,629
Corporate Bonds [Member]
Investments:
Investments		76,684	94,609
Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		76,684	94,609
Government Securities [Member]
Investments:
Investments		32,940	33,466
Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		32,940	33,466
Treasury Bonds [Member]
Investments:
Investments		16,085	17,063
Treasury Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		$ 16,085	$ 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.