Fair Value Measurements - Summary of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2013	Jan. 31, 2013
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 125,531	$ 119,911
Gross Unrealized Gains	22	83
Gross Unrealized Losses
Estimated Fair Value	125,553	119,994
Cash [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	109,721	104,109
Gross Unrealized Gains
Gross Unrealized Losses
Estimated Fair Value	109,721	104,109
Cash Equivalents [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	3,684	2,612
Gross Unrealized Gains
Gross Unrealized Losses
Estimated Fair Value	3,684	2,612
Cash and Cash Equivalents [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	113,405	106,721
Gross Unrealized Gains
Gross Unrealized Losses
Estimated Fair Value	113,405	106,721
US Government Agency Issues Short Term [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	5,018	6,043
Gross Unrealized Gains	4	61
Gross Unrealized Losses
Estimated Fair Value	5,022	6,104
US Government Agency Issues Long Term [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	7,108	7,147
Gross Unrealized Gains	18	22
Gross Unrealized Losses
Estimated Fair Value	$ 7,126	$ 7,169