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Marketable Securities with Contractual Maturities (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost less than one year $ 692,429 $ 639,923
Gross unrealized gains less than one year 3,070 3,901
Gross unrealized losses less than one year (21) (285)
Fair value less than one year 695,478 643,539
Amortized cost, over one year through five years 1,777,235 1,506,728
Gross unrealized gains, over one year through five years 15,568 17,075
Gross unrealized losses, over one year through five years (776) (3,938)
Fair value, over one year through five years 1,792,027 1,519,865
Government And Corporate Debentures Fixed Interest Rate
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost less than one year 599,614 504,749
Gross unrealized gains less than one year 2,738 3,534
Gross unrealized losses less than one year (20) (237)
Fair value less than one year 602,332 508,046
Amortized cost, over one year through five years 1,327,086 1,103,254
Gross unrealized gains, over one year through five years 13,094 14,854
Gross unrealized losses, over one year through five years (731) (2,058)
Fair value, over one year through five years 1,339,449 1,116,050
Government Sponsored Enterprises
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost less than one year 59,719 40,088
Gross unrealized gains less than one year 271 238
Fair value less than one year 59,990 40,326
Amortized cost, over one year through five years 382,758 295,235
Gross unrealized gains, over one year through five years 2,170 2,061
Gross unrealized losses, over one year through five years (39) (151)
Fair value, over one year through five years 384,889 297,145
Government And Corporate Debentures Floating Interest Rate
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost less than one year 33,096 95,086
Gross unrealized gains less than one year 61 129
Gross unrealized losses less than one year (1) (48)
Fair value less than one year 33,156 95,167
Amortized cost, over one year through five years 64,694 102,161
Gross unrealized gains, over one year through five years 304 160
Gross unrealized losses, over one year through five years (6) (1,729)
Fair value, over one year through five years 64,992 100,592
Auction Rate Securities
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost, over one year through five years 2,697 6,078
Fair value, over one year through five years $ 2,697 $ 6,078