Schedule of Available for Sale Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	$ 487,503	$ 259,011
Available-for-sale marketable securities, Unrealized Gain	1,569	592
Available-for-sale marketable securities, Unrealized (Loss)	(1,347)	(2,967)
Available-for-sale marketable securities, Fair Market Value	487,725	256,636
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	120,039	191,978
U.S. Treasury Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	191,715	110,969
Available-for-sale marketable securities, Unrealized Gain	144	112
Available-for-sale marketable securities, Unrealized (Loss)	(816)	(1,360)
Available-for-sale marketable securities, Fair Market Value	191,043	109,721
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	82,132	75,040
Commercial Paper
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	180,610	86,130
Available-for-sale marketable securities, Unrealized Gain	1	13
Available-for-sale marketable securities, Unrealized (Loss)	(7)	(26)
Available-for-sale marketable securities, Fair Market Value	180,604	86,117
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	11,237	85,094
Corporate Debt Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	90,402	41,133
Available-for-sale marketable securities, Unrealized Gain	1,343	432
Available-for-sale marketable securities, Unrealized (Loss)	(501)	(1,545)
Available-for-sale marketable securities, Fair Market Value	91,244	40,020
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	18,150	24,767
Certificates of Deposit and Time Deposits
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	11,497	7,604
Available-for-sale marketable securities, Unrealized Gain	1
Available-for-sale marketable securities, Unrealized (Loss)	(2)
Available-for-sale marketable securities, Fair Market Value	11,496	7,604
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	4,419
U.S. Government Agency Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	9,646	9,646
Available-for-sale marketable securities, Unrealized Gain	4	1
Available-for-sale marketable securities, Unrealized (Loss)	(21)	(36)
Available-for-sale marketable securities, Fair Market Value	9,629	9,611
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	4,101	7,077
Debt Mutual Funds
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	3,255	3,153
Available-for-sale marketable securities, Unrealized Gain	76	34
Available-for-sale marketable securities, Fair Market Value	3,331	3,187
Non-U.S. Government Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	378	376
Available-for-sale marketable securities, Fair Market Value	$ 378	$ 376

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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