Convertible Preferred Stock (Details) (Warrants to purchase convertible preferred stock, USD $) In Millions, unless otherwise specified	3 Months Ended
	Apr. 30, 2012
Fair value of the preferred stock warrant liability
Expense arising from revaluation	$ 14.1
Assumptions used to estimate the value of the preferred stock warrants
Dividend yield (as a percent)	0.00%
Minimum
Assumptions used to estimate the value of the preferred stock warrants
Expected volatility (as a percent)	49.70%
Risk-free rate (as a percent)	0.50%
Expected term	3 years 4 months 17 days
Maximum
Assumptions used to estimate the value of the preferred stock warrants
Expected volatility (as a percent)	53.20%
Risk-free rate (as a percent)	1.40%
Expected term	6 years 3 months 18 days