Amortized Cost, Gross Unrealized Gains (Losses) and Fair Value of Available-For-Sale Securities (Detail) - USD ($) $ in Thousands	Oct. 31, 2018	Jan. 31, 2018	Oct. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 274,604	$ 224,005	$ 134,541
Unrealized Gains		293	117
Unrealized (Losses)	(1,180)	(485)	(176)
Fair Value	273,424	223,813	134,482
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	237,916	165,337	93,307
Unrealized Gains		11	6
Unrealized (Losses)	(525)	(223)	(85)
Fair Value	237,391	165,125	93,228
Short-term Investments \| Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	200,910	111,612	67,275
Unrealized Gains			2
Unrealized (Losses)	(466)	(184)	(71)
Fair Value	200,444	111,428	67,206
Short-term Investments \| Municipal And Pre-Refunded Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	33,291	52,474	24,676
Unrealized Gains		11	4
Unrealized (Losses)	(59)	(39)	(14)
Fair Value	33,232	52,446	24,666
Short-term Investments \| Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	755	1,251	1,356
Fair Value	755	1,251	1,356
Short-term Investments \| Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,960
Fair Value	2,960
Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,688	58,668	41,234
Unrealized Gains		282	111
Unrealized (Losses)	(655)	(262)	(91)
Fair Value	36,033	58,688	41,254
Long Term Investments \| Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,793	39,853	30,051
Unrealized Gains			2
Unrealized (Losses)	(175)	(228)	(87)
Fair Value	24,618	39,625	29,966
Long Term Investments \| Municipal And Pre-Refunded Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,573	9,873	1,362
Unrealized Gains		8
Unrealized (Losses)	(7)	(24)	(2)
Fair Value	2,566	9,857	1,360
Long Term Investments \| Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,390	2,969	4,182
Fair Value	2,390	2,969	4,182
Long Term Investments \| Mutual Funds, Held in Rabbi Trust
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,932	5,973	5,639
Unrealized Gains		274	109
Unrealized (Losses)	(473)	(10)	(2)
Fair Value	$ 6,459	$ 6,237	$ 5,746

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) 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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-Publisher FASB
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-Topic 320
-SubTopic 10
-Section 50
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) 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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Amortized cost 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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Amount 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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