Schedule of Available for Sale Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	$ 259,011	$ 1,470,729
Available-for-sale marketable securities, Unrealized Gain	592	6,582
Available-for-sale marketable securities, Unrealized (Loss)	(2,967)	(3,406)
Available-for-sale marketable securities, Fair Market Value	256,636	1,473,905
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	191,978	1,350,896
U.S. Treasury Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	110,969	858,258
Available-for-sale marketable securities, Unrealized Gain	112	72
Available-for-sale marketable securities, Unrealized (Loss)	(1,360)	(2,535)
Available-for-sale marketable securities, Fair Market Value	109,721	855,795
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	75,040	850,163
Commercial Paper
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	86,130	283,009
Available-for-sale marketable securities, Unrealized Gain	13	18
Available-for-sale marketable securities, Unrealized (Loss)	(26)	(187)
Available-for-sale marketable securities, Fair Market Value	86,117	282,840
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	85,094	258,933
Corporate Debt Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	41,133	131,179
Available-for-sale marketable securities, Unrealized Gain	432	2,380
Available-for-sale marketable securities, Unrealized (Loss)	(1,545)	(373)
Available-for-sale marketable securities, Fair Market Value	40,020	133,186
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	24,767	91,010
U.S. Government Agency Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	9,646	10,775
Available-for-sale marketable securities, Unrealized Gain	1
Available-for-sale marketable securities, Unrealized (Loss)	(36)	(49)
Available-for-sale marketable securities, Fair Market Value	9,611	10,726
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	7,077	10,727
Certificates of Deposit and Time Deposits
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	7,604	167,523
Available-for-sale marketable securities, Unrealized Gain		6
Available-for-sale marketable securities, Unrealized (Loss)		(187)
Available-for-sale marketable securities, Fair Market Value	7,604	167,342
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses		138,340
Debt Mutual Funds
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	3,153
Available-for-sale marketable securities, Unrealized Gain	34
Available-for-sale marketable securities, Fair Market Value	3,187
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	0
Non-U.S. Government Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	376	582
Available-for-sale marketable securities, Unrealized Gain		4
Available-for-sale marketable securities, Fair Market Value	$ 376	586
Equity and Debt Mutual Funds
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost		19,403
Available-for-sale marketable securities, Unrealized Gain		4,102
Available-for-sale marketable securities, Unrealized (Loss)		(75)
Available-for-sale marketable securities, Fair Market Value		23,430
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses		$ 1,723

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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