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Investments with Continuous Unrealized Losses (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Investments with continuous unrealized losses for less than 12 months, Fair value $ 1,911,338 $ 2,184,292
Investments with continuous unrealized losses for less than 12 months, Unrealized losses (8,636) (15,554)
Investments with continuous unrealized losses for 12 months or greater, Fair value 1,358,676 45,464
Investments with continuous unrealized losses for 12 months or greater, Unrealized losses (13,266) (31)
Total Investments with continuous unrealized losses, Fair Value 3,270,014 2,229,756
Total Investments with continuous unrealized losses, Unrealized losses (21,902) (15,585)
Government And Corporate Debentures Fixed Interest Rate    
Schedule of Available-for-sale Securities [Line Items]    
Investments with continuous unrealized losses for less than 12 months, Fair value 1,647,125 1,717,212
Investments with continuous unrealized losses for less than 12 months, Unrealized losses (7,318) (11,440)
Investments with continuous unrealized losses for 12 months or greater, Fair value 1,063,690 40,465
Investments with continuous unrealized losses for 12 months or greater, Unrealized losses (9,585) (30)
Total Investments with continuous unrealized losses, Fair Value 2,710,815 1,757,677
Total Investments with continuous unrealized losses, Unrealized losses (16,903) (11,470)
Government Sponsored Enterprises Debentures    
Schedule of Available-for-sale Securities [Line Items]    
Investments with continuous unrealized losses for less than 12 months, Fair value 236,403 464,080
Investments with continuous unrealized losses for less than 12 months, Unrealized losses (1,281) (4,112)
Investments with continuous unrealized losses for 12 months or greater, Fair value 294,986 4,999
Investments with continuous unrealized losses for 12 months or greater, Unrealized losses (3,681) (1)
Total Investments with continuous unrealized losses, Fair Value 531,389 469,079
Total Investments with continuous unrealized losses, Unrealized losses (4,962) (4,113)
Government And Corporate Debentures Floating Interest Rate    
Schedule of Available-for-sale Securities [Line Items]    
Investments with continuous unrealized losses for less than 12 months, Fair value 27,810 3,000
Investments with continuous unrealized losses for less than 12 months, Unrealized losses (37) (2)
Total Investments with continuous unrealized losses, Fair Value 27,810 3,000
Total Investments with continuous unrealized losses, Unrealized losses $ (37) $ (2)