Fair Value Measurements - Information about Assets that are Measured at Fair Value on a Recurring Basis (Detail) (USD $) | Apr. 30, 2014 | Jan. 31, 2014 |
In Thousands, unless otherwise specified |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | $1,808,835 | $1,827,175 |
U.S. agency obligations [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 1,403,420 | 1,125,454 |
U.S. treasury securities [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 214,858 | 536,788 |
Commercial paper [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 106,982 | 62,997 |
U.S. corporate securities [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 31,680 | 11,777 |
Money market funds [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 51,895 | 90,159 |
Included in cash and cash equivalents [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 188,885 | 521,922 |
Included in marketable securities [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 1,619,950 | 1,305,253 |
Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 266,753 | 626,947 |
Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. agency obligations [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | ' | ' |
Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. treasury securities [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 214,858 | 536,788 |
Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Commercial paper [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | ' | ' |
Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. corporate securities [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | ' | ' |
Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money market funds [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 51,895 | 90,159 |
Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 1,542,082 | 1,200,228 |
Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | U.S. agency obligations [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 1,403,420 | 1,125,454 |
Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | U.S. treasury securities [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | ' | ' |
Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commercial paper [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 106,982 | 62,997 |
Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | U.S. corporate securities [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | 31,680 | 11,777 |
Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Money market funds [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Assets Fair Value Disclosure | ' | ' |