Cash, Cash Equivalents and Investments - Gross Unrealized Losses and Fair Value for Available-for-Sale Investments (Detail) $ in Thousands	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	$ 294,355
Unrealized Losses, Less than 12 months	(814)
Fair Value, Greater than 12 Months	20,929
Unrealized Losses, Greater than 12 Months	(34)
Fair Value	315,284
Unrealized Losses	(848)
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	117,853
Unrealized Losses, Less than 12 months	(400)
Fair Value, Greater than 12 Months	14,931
Unrealized Losses, Greater than 12 Months	(30)
Fair Value	132,784
Unrealized Losses	(430)
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	99,358
Unrealized Losses, Less than 12 months	(199)
Fair Value	99,358
Unrealized Losses	(199)
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	45,429
Unrealized Losses, Less than 12 months	(70)
Fair Value, Greater than 12 Months	5,998
Unrealized Losses, Greater than 12 Months	(4)
Fair Value	51,427
Unrealized Losses	(74)
Sovereign Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	27,687
Unrealized Losses, Less than 12 months	(140)
Fair Value, Greater than 12 Months	0
Unrealized Losses, Greater than 12 Months	0
Fair Value	27,687
Unrealized Losses	(140)
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	4,028
Unrealized Losses, Less than 12 months	(5)
Unrealized Losses, Greater than 12 Months	0
Fair Value	4,028
Unrealized Losses	$ (5)