Financial Instruments - Estimated Fair Value and Related Valuation Input Hierarchy of Financial Assets Comprised Solely of Available-for-Sale Securities Measured on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	$ 319,513	$ 104,216
Total Unrealized Gain	11	3
Total Unrealized Loss	(530)	(103)
Total Fair Value	318,994	104,116
Money Market Funds | Level 1
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	16,364	18,141
Total Fair Value	16,364	18,141
U.S. Treasury Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	599	466
Total Unrealized Loss	(1)	(1)
Total Fair Value	598	465
Government Agency Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	36,480	12,519
Total Unrealized Gain	1
Total Unrealized Loss	(88)	(5)
Total Fair Value	36,393	12,514
Corporate Debt Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	203,767	60,052
Total Unrealized Gain	8	1
Total Unrealized Loss	(339)	(89)
Total Fair Value	203,436	59,964
Commercial Paper | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	999	1,200
Total Fair Value	999	1,200
Asset-Backed Securities | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	61,304	11,838
Total Unrealized Gain	2	2
Total Unrealized Loss	(102)	(8)
Total Fair Value	61,204	11,832
Amounts Classified As Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	22,259	21,897
Total Unrealized Loss	(1)
Total Fair Value	22,258	21,897
Amounts Classified As Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	297,254	82,319
Total Unrealized Gain	11	3
Total Unrealized Loss	(529)	(103)
Total Fair Value	$ 296,736	$ 82,219