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Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 31, 2019	Oct. 31, 2018	Oct. 31, 2019	Oct. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Sales and maturities of marketable securities			$ 382,629	$ 211,174
Amortization of discounts and premiums, net	$ 156	$ 328	427	1,479
Other Income, Net
Schedule of Available-for-sale Securities [Line Items]
Net realized gain (loss)	$ 6	$ (3)	$ 32	$ (16)

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Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of cash inflow from sale, maturity, prepayment and call of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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