Schedule of Available for Sale Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	$ 488,532	$ 1,470,729
Available-for-sale marketable securities, Unrealized Gain	544	6,582
Available-for-sale marketable securities, Unrealized (Loss)	(3,369)	(3,406)
Available-for-sale marketable securities, Fair Market Value	485,707	1,473,905
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	461,349	1,350,896
U.S. Treasury Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	365,701	858,258
Available-for-sale marketable securities, Unrealized Gain		72
Available-for-sale marketable securities, Unrealized (Loss)	(2,287)	(2,535)
Available-for-sale marketable securities, Fair Market Value	363,414	855,795
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	362,167	850,163
Commercial Paper
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	67,208	283,009
Available-for-sale marketable securities, Unrealized Gain	7	18
Available-for-sale marketable securities, Unrealized (Loss)	(13)	(187)
Available-for-sale marketable securities, Fair Market Value	67,202	282,840
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	66,186	258,933
Corporate Debt Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	40,669	131,179
Available-for-sale marketable securities, Unrealized Gain	537	2,380
Available-for-sale marketable securities, Unrealized (Loss)	(935)	(373)
Available-for-sale marketable securities, Fair Market Value	40,271	133,186
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	22,030	91,010
U.S. Government Agency Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	9,065	10,775
Available-for-sale marketable securities, Unrealized (Loss)	(61)	(49)
Available-for-sale marketable securities, Fair Market Value	9,004	10,726
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	9,004	10,726
Debt Mutual Funds
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	3,128
Available-for-sale marketable securities, Unrealized (Loss)	(73)
Available-for-sale marketable securities, Fair Market Value	3,055
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	1,786
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
Certificates of Deposit and Time Deposits
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	2,218	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	2,218	167,342
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses		138,340
Non-U.S. Government Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	543	582
Available-for-sale marketable securities, Unrealized Gain		4
Available-for-sale marketable securities, Fair Market Value	543	$ 586
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	$ 176