Balance Sheet Components - Summary of Fair Value and Gross Unrealized Gains and Losses on Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 146,796	$ 767,898
Gross unrealized gains	138	379
Gross unrealized losses		(79)
Fair Value, Total	146,934	768,198
U.S. government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	116,196	304,897
Gross unrealized gains	121	131
Gross unrealized losses		(7)
Fair Value, Total	116,317	305,021
U.S. government agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,699	125,904
Gross unrealized gains	17	35
Gross unrealized losses		(22)
Fair Value, Total	17,716	125,917
Corporate bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,001	190,068
Gross unrealized gains		149
Gross unrealized losses		(40)
Fair Value, Total	1,001	190,177
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,520	76,126
Gross unrealized gains		27
Gross unrealized losses		(1)
Fair Value, Total	9,520	76,152
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,380	1,989
Gross unrealized gains		4
Fair Value, Total	2,380	1,993
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		68,914
Gross unrealized gains		33
Gross unrealized losses		(9)
Fair Value, Total		$ 68,938