Fair Value of Financial Assets and Liabilities - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Assets:
Cash Equivalents		$ 25,964	$ 6,307
Repurchase Agreements			7,000
Investments:
Total assets		139,859	188,763
Fair Value, Inputs, Level 2 [Member]
Assets:
Cash Equivalents			4,740
Repurchase Agreements			7,000
Investments:
Total assets		113,895	187,196
Not Subject to Leveling [Member]
Assets:
Cash Equivalents	[1]	25,964	1,567
Investments:
Total assets	[1]	25,964	1,567
Commercial Paper [Member]
Investments:
Investments		6,198	19,689
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		6,198	19,689
Certificates of Deposit [Member]
Investments:
Investments		8,916	10,629
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		8,916	10,629
Corporate Bonds [Member]
Investments:
Investments		58,865	94,609
Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		58,865	94,609
Government Securities [Member]
Investments:
Investments		22,954	33,466
Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		22,954	33,466
Treasury Bonds [Member]
Investments:
Investments		16,962	17,063
Treasury Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		$ 16,962	$ 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.