Schedule of Available for Sale Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	$ 1,470,729	$ 1,304,711
Available-for-sale marketable securities, Unrealized Gain	6,582	2,977
Available-for-sale marketable securities, Unrealized (Loss)	(3,406)	(2,821)
Available-for-sale marketable securities, Fair Market Value	1,473,905	1,304,867
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	1,350,896	732,979
U.S. Treasury Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	858,258	904,737
Available-for-sale marketable securities, Unrealized Gain	72	97
Available-for-sale marketable securities, Unrealized (Loss)	(2,535)	(2,034)
Available-for-sale marketable securities, Fair Market Value	855,795	902,800
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	850,163	572,284
Corporate Debt Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	131,179	99,708
Available-for-sale marketable securities, Unrealized Gain	2,380	1,065
Available-for-sale marketable securities, Unrealized (Loss)	(373)	(620)
Available-for-sale marketable securities, Fair Market Value	133,186	100,153
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	91,010	53,642
U.S. Government Agency Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	10,775	39,264
Available-for-sale marketable securities, Unrealized Gain		7
Available-for-sale marketable securities, Unrealized (Loss)	(49)	(19)
Available-for-sale marketable securities, Fair Market Value	10,726	39,252
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	10,726	13,461
Certificates of Deposit and Time Deposits
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	167,523	82,080
Available-for-sale marketable securities, Unrealized Gain	6	54
Available-for-sale marketable securities, Unrealized (Loss)	(187)	(1)
Available-for-sale marketable securities, Fair Market Value	167,342	82,133
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	138,340	7,760
Commercial Paper
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	283,009	161,672
Available-for-sale marketable securities, Unrealized Gain	18	24
Available-for-sale marketable securities, Unrealized (Loss)	(187)	(66)
Available-for-sale marketable securities, Fair Market Value	282,840	161,630
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	258,933	84,034
Equity and Debt Mutual Funds
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	19,403	16,505
Available-for-sale marketable securities, Unrealized Gain	4,102	1,724
Available-for-sale marketable securities, Unrealized (Loss)	(75)	(58)
Available-for-sale marketable securities, Fair Market Value	23,430	18,171
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses	1,723	1,661
Non-U.S. Government Securities
Schedule of Available-for-sale Securities
Available-for-sale marketable securities, Cost	582	745
Available-for-sale marketable securities, Unrealized Gain	4	6
Available-for-sale marketable securities, Unrealized (Loss)		(23)
Available-for-sale marketable securities, Fair Market Value	$ 586	728
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses		$ 137

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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