Amortized Cost, Gross Unrealized Gains (Losses) and Fair Value of Available-For-Sale Securities (Detail) - USD ($) $ in Thousands	Jul. 31, 2018	Jan. 31, 2018	Jul. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 244,023	$ 224,005	$ 136,122
Unrealized Gains	182	293	131
Unrealized (Losses)	(525)	(485)	(98)
Fair Value	243,680	223,813	136,155
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	198,460	165,337	110,250
Unrealized Gains	10	11	19
Unrealized (Losses)	(304)	(223)	(74)
Fair Value	198,166	165,125	110,195
Short-term Investments \| Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	150,648	111,612	72,805
Unrealized Gains			4
Unrealized (Losses)	(261)	(184)	(61)
Fair Value	150,387	111,428	72,748
Short-term Investments \| Municipal And Pre-Refunded Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,904	52,474	36,630
Unrealized Gains	10	11	15
Unrealized (Losses)	(43)	(39)	(13)
Fair Value	46,871	52,446	36,632
Short-term Investments \| Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	908	1,251	815
Fair Value	908	1,251	815
Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,563	58,668	25,872
Unrealized Gains	172	282	112
Unrealized (Losses)	(221)	(262)	(24)
Fair Value	45,514	58,688	25,960
Long Term Investments \| Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	34,333	39,853	14,906
Unrealized (Losses)	(213)	(228)	(22)
Fair Value	34,120	39,625	14,884
Long Term Investments \| Municipal And Pre-Refunded Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,130	9,873	3,057
Unrealized Gains	1	8	9
Unrealized (Losses)	(7)	(24)	(2)
Fair Value	2,124	9,857	3,064
Long Term Investments \| Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,699	2,969	2,726
Fair Value	2,699	2,969	2,726
Long Term Investments \| Mutual Funds, Held in Rabbi Trust
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,401	5,973	5,183
Unrealized Gains	171	274	103
Unrealized (Losses)	(1)	(10)
Fair Value	$ 6,571	$ 6,237	$ 5,286

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
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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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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-Publisher FASB
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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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