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Notes Payable - Recognized Amounts in Interest Expense in the Condensed Consolidated Statements of Income Related to the 4.00% Debentures (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Oct. 31, 2016
Oct. 31, 2015
Oct. 31, 2016
Oct. 31, 2015
Debt Instrument        
Interest expense at the contractual interest rate $ 5,143 $ 4,915 $ 14,971 $ 14,381
4.00% Debentures due 2031        
Debt Instrument        
Interest expense at the contractual interest rate 2,530 2,530 7,589 7,590
Amortization of debt discount 1,786 1,663 5,263 4,900
Amortization of debt issuance costs $ 238 $ 238 $ 714 $ 714