Fair Value Measurements (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2011
Fair Value Measurements
Money market funds	$ 250,800	$ 19,500	$ 13,100
Preferred stock warrants		(2,133)
Assets:
Restricted cash		514	600
Total	250,800	19,500	13,100
Liabilities:
Preferred stock warrants		(2,133)
Net asset value per share of money market funds (in dollars per share)	$ 1
Recurring basis | Level 1
Fair Value Measurements
Money market funds	250,810	20,142
Assets:
Total	250,810	20,142
Recurring basis | Level 3
Fair Value Measurements
Preferred stock warrants		(2,133)
Liabilities:
Preferred stock warrants		(2,133)
Recurring basis | Total
Fair Value Measurements
Money market funds	250,810	20,142
Preferred stock warrants		(2,133)
Assets:
Cash and cash equivalents	250,810	19,499
Prepaid expenses and other current assets		129
Restricted cash		514
Total	250,810	20,142
Liabilities:
Preferred stock warrants		$ (2,133)