Financial Assets and Liabilities - Short-term investments (Details) (USD $) In Thousands, unless otherwise specified	Jan. 31, 2015
Cash equivalents
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	$ 388,467
Gross unrealized gains	0
Gross unrealized losses less than 12 months	0
Gross unrealized losses 12 months or longer	0
Estimated fair value	388,467	[1]
Cash equivalents | Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	2,400
Gross unrealized gains	0
Gross unrealized losses less than 12 months	0
Gross unrealized losses 12 months or longer	0
Estimated fair value	2,400	[1]
Cash equivalents | Money market funds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	377,370
Gross unrealized gains	0
Gross unrealized losses less than 12 months	0
Gross unrealized losses 12 months or longer	0
Estimated fair value	377,370	[1]
Cash equivalents | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	8,697
Gross unrealized gains	0
Gross unrealized losses less than 12 months	0
Gross unrealized losses 12 months or longer	0
Estimated fair value	8,697	[1]
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	120,221
Gross unrealized gains	39
Gross unrealized losses less than 12 months	(22)
Gross unrealized losses 12 months or longer	0
Estimated fair value	120,238	[1]
Short-term investments | Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	3,400
Gross unrealized gains	0
Gross unrealized losses less than 12 months	0
Gross unrealized losses 12 months or longer	0
Estimated fair value	3,400	[1]
Short-term investments | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	7,090
Gross unrealized gains	0
Gross unrealized losses less than 12 months	0
Gross unrealized losses 12 months or longer	0
Estimated fair value	7,090	[1]
Short-term investments | U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	17,928
Gross unrealized gains	8
Gross unrealized losses less than 12 months	0
Gross unrealized losses 12 months or longer	0
Estimated fair value	17,936	[1]
Short-term investments | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	1,405
Gross unrealized gains	0
Gross unrealized losses less than 12 months	(2)
Gross unrealized losses 12 months or longer	0
Estimated fair value	1,403	[1]
Short-term investments | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	67,160
Gross unrealized gains	26
Gross unrealized losses less than 12 months	(18)
Gross unrealized losses 12 months or longer	0
Estimated fair value	67,168	[1]
Short-term investments | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	23,238
Gross unrealized gains	5
Gross unrealized losses less than 12 months	(2)
Gross unrealized losses 12 months or longer	0
Estimated fair value	$ 23,241	[1]

[1] See Note 5. Fair Value Measures for further discussion on fair values of cash equivalents and short-term investments.