Cash, Cash Equivalents and Investments - Gross Unrealized Losses and Fair Value for Available-for-Sale Investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	$ 244,230
Unrealized Losses, Less than 12 months	(664)
Fair Value, Greater than 12 Months	3,487
Unrealized Losses, Greater than 12 Months	(16)
Fair Value	247,717
Unrealized Losses	(680)
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	126,038
Unrealized Losses, Less than 12 months	(555)
Fair Value, Greater than 12 Months	0
Unrealized Losses, Greater than 12 Months	0
Fair Value	126,038
Unrealized Losses	(555)
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	70,901
Unrealized Losses, Less than 12 months	(54)
Fair Value, Greater than 12 Months	0
Unrealized Losses, Greater than 12 Months	0
Fair Value	70,901
Unrealized Losses	(54)
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	33,603
Unrealized Losses, Less than 12 months	(36)
Fair Value, Greater than 12 Months	3,487
Unrealized Losses, Greater than 12 Months	(16)
Fair Value	37,090
Unrealized Losses	(52)
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	7,135
Unrealized Losses, Less than 12 months	(8)
Fair Value, Greater than 12 Months	0
Unrealized Losses, Greater than 12 Months	0
Fair Value	7,135
Unrealized Losses	(8)
Sovereign Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	6,553
Unrealized Losses, Less than 12 months	(11)
Fair Value, Greater than 12 Months	0
Unrealized Losses, Greater than 12 Months	0
Fair Value	6,553
Unrealized Losses	$ (11)