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Gross Unrealized Losses and Fair Value of Marketable Securities (Detail) - USD ($)
$ in Thousands
Jan. 31, 2020
Jan. 31, 2019
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value $ 93,446 $ 204,953
Less Than 12 Months, Unrealized Losses (151) (175)
12 Months or Greater, Fair Value   43,108
12 Months or Greater, Unrealized Losses   (125)
Total, Fair Value 93,446 248,061
Total, Unrealized Losses (151) (300)
Corporate Bonds    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value 60,067 177,932
Less Than 12 Months, Unrealized Losses (66) (160)
12 Months or Greater, Fair Value   38,201
12 Months or Greater, Unrealized Losses   (117)
Total, Fair Value 60,067 216,133
Total, Unrealized Losses (66) (277)
Municipal And Pre-Refunded Municipal Bonds    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value 24,975 20,732
Less Than 12 Months, Unrealized Losses (28) (13)
12 Months or Greater, Fair Value   4,907
12 Months or Greater, Unrealized Losses   (8)
Total, Fair Value 24,975 25,639
Total, Unrealized Losses (28) (21)
Mutual Funds, Held in Rabbi Trust    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value 5,395  
Less Than 12 Months, Unrealized Losses (55)  
Total, Fair Value 5,395  
Total, Unrealized Losses (55)  
Federal Government Agencies    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value 3,009 3,315
Less Than 12 Months, Unrealized Losses (2) (1)
Total, Fair Value 3,009 3,315
Total, Unrealized Losses $ (2) (1)
Treasury Bills    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value   2,974
Less Than 12 Months, Unrealized Losses   (1)
Total, Fair Value   2,974
Total, Unrealized Losses   $ (1)