Stock-based Compensation Expense Stock-Based Compensation Expense - Additional Information (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended				12 Months Ended
	Apr. 30, 2012 Year		Apr. 30, 2011		Jan. 31, 2012
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share based compensation arragements by share based payment awards, number of active plans (presented as an integer)	2
Share based compensation arrangement by share based payment award expired or terminated plans (presented as an integer)	8
Share-Based Compensation Shares Granted to Share Issued	$ 2.11
EmployeeStockPlanRestrictedStockSharesRemainingAvailableForIssuance	2,600,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized (in usd)	$ 65.2
Share Based Compensation Non Vested Options Outstanding Weighted Average Remaining Contractual Term (in years)	1.8
Sale of Stock, Price Per Share (in usd per share)	$ 39.37
Employee Service Share Based Compensation Unrecognized Compensation Costs On Nonvested Restricted Shares (in usd)	65.2
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value (in usd per share)	$ 13.43	[1]	$ 14.40	[1]
Employee Stock Plan 2012 [Member]
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	21,200,000
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures	15,200,000
Stock Issued During Period, Shares, Share-based Compensation, Forfeited	6,000,000
Share-Based Compensation Shares Granted to Share Issued	$ 1.79
Share Based Compensation Arrangement By Share Based Payment Award Maximum Vesting Period For Stock Options (in years)	over a three year period
Employee Stock Plan Shares Remaining Available For Issuance	13,900,000
Employee Stock Plan 2012 [Member] | Maximum [Member]
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share based compensation arrangement by share based payment award options expiration term (in years)	10
Outside directors stock plan 2012 [Member]
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	2,600,000
Employee Stock Plan Shares Remaining Available For Issuance	2,600,000
Employee Qualified Stock Purchase Plan 1998 ESP Plan [Member]
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Employee Stock Plan Shares Remaining Available For Issuance	32,900,000
Employee Stock Purchase Plan Maximum Percentage Of Compensation To Purchase Shares By Eligible Participants (presented as a percent)	15.00%
Employee Stock Purchase Plan Minimum Percentage Of Common Stock Fair Value Defined To Purchase Shares By Eligible Participants (presented as a percent)	85.00%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Other Increases (Decreases) in Period	10,000,000
Share Based Compensation Arrangement By Share Based Payment Award Options Other Increases Decreases In Period Percentage Calculation (presented as a percent)	2.00%
Stock Issued During Period, Shares, Employee Stock Purchase Plans	1,600,000		1,800,000
Share Based Compensation Arrangement By Share Based Payment Award Shares Issued Average Price (in usd per share)	$ 21.63		$ 15.28
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value (in usd per share)	$ 14.00		$ 13.46
Restricted Stock Units (RSUs) [Member]
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized (in usd)	57.4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	600,000
Allocated Share-based Compensation Expense (in usd)	10.8		5.3
Employee Service Share Based Compensation Unrecognized Compensation Costs On Nonvested Restricted Shares (in usd)	57.4
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition (in years)	1.6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	2,500,000
Performance Share [Member]
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized (in usd)	12.4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	500,000
Allocated Share-based Compensation Expense (in usd)	1.2		0
Employee Service Share Based Compensation Unrecognized Compensation Costs On Nonvested Restricted Shares (in usd)	12.4
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition (in years)	1.9
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	500,000
Restricted stock awards [member]
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized (in usd)	0.1
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period					0
Allocated Share-based Compensation Expense (in usd)	0.1		0.1
Employee Service Share Based Compensation Unrecognized Compensation Costs On Nonvested Restricted Shares (in usd)	$ 0.1
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition (in years)	0.1
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	13,500

[1] The weighted average grant date fair value of stock options granted is calculated, as of the stock option grant date, using the Black-Scholes-Merton option pricing model.