Amortized Cost, Gross Unrealized Gains (Losses) and Fair Value of Available-For-Sale Securities (Detail) - USD ($) $ in Thousands	Oct. 31, 2015	Jan. 31, 2015	Oct. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 123,316	$ 208,587	$ 229,342
Unrealized Gains	444	233	387
Unrealized (Losses)	(77)	(126)	(124)
Fair Value	123,683	208,694	229,605
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	69,541	104,201	114,946
Unrealized Gains	50	71	89
Unrealized (Losses)	(46)	(26)	(30)
Fair Value	69,545	104,246	115,005
Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	53,775	104,386	114,396
Unrealized Gains	394	162	298
Unrealized (Losses)	(31)	(100)	(94)
Fair Value	54,138	104,448	114,600
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	52,408	103,326	130,362
Corporate Bonds | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,260	56,594	70,859
Unrealized Gains	4	20	47
Unrealized (Losses)	(45)	(24)	(30)
Fair Value	37,219	56,590	70,876
Corporate Bonds | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,210	46,754	59,504
Unrealized Gains	7	22	65
Unrealized (Losses)	(28)	(40)	(83)
Fair Value	15,189	46,736	59,486
Municipal And Pre-Refunded Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	61,523	73,495	65,155
Municipal And Pre-Refunded Municipal Bonds | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31,100	30,509	26,746
Unrealized Gains	46	41	34
Unrealized (Losses)	(1)	(2)
Fair Value	31,145	30,548	26,780
Municipal And Pre-Refunded Municipal Bonds | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,265	42,840	38,292
Unrealized Gains	115	113	90
Unrealized (Losses)	(2)	(6)	(7)
Fair Value	30,378	42,947	38,375
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Fair Value	4,641	14,198	15,668
Certificates of Deposit | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,181	11,127	11,978
Unrealized Gains		5	4
Fair Value	1,181	11,132	11,982
Certificates of Deposit | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,460	3,066	3,686
Fair Value	3,460	3,066	3,686
Treasury Bills
Schedule of Available-for-sale Securities [Line Items]
Fair Value		9,156	8,726
Treasury Bills | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,033
Unrealized Gains		3
Fair Value		2,036
Treasury Bills | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		7,111	8,717
Unrealized Gains		9	9
Fair Value		7,120	8,726
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Fair Value		3,940	3,593
Commercial Paper | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		3,938	3,592
Unrealized Gains		2	1
Fair Value		3,940	3,593
Mutual Funds, Held in Rabbi Trust
Schedule of Available-for-sale Securities [Line Items]
Fair Value	5,111	3,778	3,529
Mutual Funds, Held in Rabbi Trust | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,840	3,816	3,399
Unrealized Gains	272	16	134
Unrealized (Losses)	(1)	(54)	(4)
Fair Value	$ 5,111	3,778	3,529
Federal Government Agencies
Schedule of Available-for-sale Securities [Line Items]
Fair Value		801	2,572
Federal Government Agencies | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			1,771
Unrealized Gains			3
Fair Value			1,774
Federal Government Agencies | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		799	798
Unrealized Gains		2
Fair Value		$ 801	$ 798