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Gross Unrealized Losses and Fair Value of Marketable Securities (Detail) - USD ($)
$ in Thousands
Jan. 31, 2019
Jan. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value $ 204,953 $ 191,198
Less Than 12 Months, Unrealized Losses (175) (485)
12 Months or Greater, Fair Value 43,108  
12 Months or Greater, Unrealized Losses (125)  
Total, Fair Value 248,061 191,198
Total, Unrealized Losses (300) (485)
Corporate Bonds    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value 177,932 151,053
Less Than 12 Months, Unrealized Losses (160) (412)
12 Months or Greater, Fair Value 38,201  
12 Months or Greater, Unrealized Losses (117)  
Total, Fair Value 216,133 151,053
Total, Unrealized Losses (277) (412)
Municipal And Pre-Refunded Municipal Bonds    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value 20,732 39,671
Less Than 12 Months, Unrealized Losses (13) (63)
12 Months or Greater, Fair Value 4,907  
12 Months or Greater, Unrealized Losses (8)  
Total, Fair Value 25,639 39,671
Total, Unrealized Losses (21) (63)
Federal Government Agencies    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value 3,315  
Less Than 12 Months, Unrealized Losses (1)  
Total, Fair Value 3,315  
Total, Unrealized Losses (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)  
Mutual Funds, Held in Rabbi Trust    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value   474
Less Than 12 Months, Unrealized Losses   (10)
Total, Fair Value   474
Total, Unrealized Losses   $ (10)