Balance Sheet Components - Net Unrealized Gains (Losses) on Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 17,667	$ 530,006
Gross unrealized gains	0	75
Gross unrealized losses	(208)	(247)
Fair Value	17,459	529,834
U.S. government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		336,561
Gross unrealized gains		45
Gross unrealized losses		(166)
Fair Value	0	336,440
U.S. government agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		193,006
Gross unrealized gains		30
Gross unrealized losses		(81)
Fair Value	0	192,955
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,106	439
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair Value	14,106	439
Publicly traded equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,561
Gross unrealized gains	0
Gross unrealized losses	(208)
Fair Value	$ 3,353	$ 0