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Marketable Securities with Contractual Maturities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost up to one year $ 1,448,859 $ 1,167,187
Gross unrealized gains up to one year 15 63
Gross unrealized losses up to one year (6,232) (1,984)
Fair value up to one year 1,442,642 1,165,266
Amortized cost, over one year through five years 2,312,609 2,456,715
Gross unrealized gains, over one year through five years 1,368 518
Gross unrealized losses, over one year through five years (26,632) (19,918)
Fair value, over one year through five years 2,287,345 2,437,315
Government And Corporate Debentures Fixed Interest Rate    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost up to one year 1,203,258 1,026,555
Gross unrealized gains up to one year 9 48
Gross unrealized losses up to one year (5,017) (1,698)
Fair value up to one year 1,198,249 1,024,905
Amortized cost, over one year through five years 1,819,541 1,985,849
Gross unrealized gains, over one year through five years 1,089 384
Gross unrealized losses, over one year through five years (21,630) (15,204)
Fair value, over one year through five years 1,799,001 1,971,029
Government Sponsored Enterprises    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost up to one year 231,699 112,951
Gross unrealized gains up to one year 6 1
Gross unrealized losses up to one year (1,200) (275)
Fair value up to one year 230,505 112,677
Amortized cost, over one year through five years 383,190 424,619
Gross unrealized gains, over one year through five years 278  
Gross unrealized losses, over one year through five years (4,092) (4,687)
Fair value, over one year through five years 379,376 419,932
Government And Corporate Debentures Floating Interest Rate    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost up to one year 13,902 27,681
Gross unrealized gains up to one year   14
Gross unrealized losses up to one year (15) (11)
Fair value up to one year 13,887 27,684
Amortized cost, over one year through five years 109,878 46,247
Gross unrealized gains, over one year through five years 1 134
Gross unrealized losses, over one year through five years (910) (27)
Fair value, over one year through five years $ 108,969 $ 46,354