Cash, Cash Equivalents and Investments - Amortized Cost to Fair Value (Detail) $ in Thousands	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 665,464
Gross Unrealized Gains	913
Gross Unrealized Losses	(848)
Fair Value, Total	665,529
Euro liquidity fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	46,499
Euro liquidity fund [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,173
Gross Unrealized Gains	326
Gross Unrealized Losses	0
Fair Value, Total	46,499
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	311,140
Corporate Bonds [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	141,216
Gross Unrealized Gains	37
Gross Unrealized Losses	(65)
Fair Value, Total	141,188
Corporate Bonds [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	169,911
Gross Unrealized Gains	406
Gross Unrealized Losses	(365)
Fair Value, Total	169,952
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	13,242
Agency Bonds [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,900
Gross Unrealized Gains	3
Gross Unrealized Losses	0
Fair Value, Total	2,903
Agency Bonds [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,310
Gross Unrealized Gains	29
Gross Unrealized Losses	0
Fair Value, Total	10,339
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	96,560
Asset-Backed Securities [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	69,623
Gross Unrealized Gains	18
Gross Unrealized Losses	(27)
Fair Value, Total	69,614
Asset-Backed Securities [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,968
Gross Unrealized Gains	25
Gross Unrealized Losses	(47)
Fair Value, Total	26,946
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	7,750
Municipal Bonds [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,517
Gross Unrealized Gains	2
Gross Unrealized Losses	(2)
Fair Value, Total	6,517
Municipal Bonds [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,236
Gross Unrealized Gains	0
Gross Unrealized Losses	(3)
Fair Value, Total	1,233
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	159,455
Treasury Bills [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	67,201
Gross Unrealized Gains	21
Gross Unrealized Losses	(47)
Fair Value, Total	67,175
Treasury Bills [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	92,392
Gross Unrealized Gains	40
Gross Unrealized Losses	(152)
Fair Value, Total	92,280
Sovereign Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	30,883
Sovereign Bonds [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,313
Gross Unrealized Gains	0
Gross Unrealized Losses	(15)
Fair Value, Total	7,298
Sovereign Bonds [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,704
Gross Unrealized Gains	6
Gross Unrealized Losses	(125)
Fair Value, Total	$ 23,585