Share-Based Compensation (Share-Based Compensation Activity - Stock Options) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Beginning Balance, Shares	5.5
Exercised, Shares	(2.2)
Ending Balance, Shares	3.3	5.5
Vested and exercisable, Shares	3.3
Beginning Balance, Weighted-Average Exercise Price	$ 30
Exercised, Weighted-Average Exercise Price	$ 30
Ending Balance, Weighted-Average Exercise Price	$ 31	$ 30
Vested and exercisable, Weighted-Average Exercise Price	$ 31
Beginning Balance, Weighted-Average Remaining Contractual Term	1 year 4 months 24 days	2 years 1 month 6 days
Ending Balance, Weighted-Average Remaining Contractual Term	1 year 4 months 24 days	2 years 1 month 6 days
Vested and exercisable, Weighted-Average Remaining Contractual Term	1 year 4 months 24 days
Beginning Balance, Aggregate Intrinsic Value	$ 57.1
Ending Balance, Aggregate Intrinsic Value	52.6	57.1
Vested and exercisable, Aggregate Intrinsic Value	$ 52.5

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of options outstanding.

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The number of shares reserved for issuance under stock option agreements awarded under the plan that validly exist and are outstanding as of the balance sheet date, including vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of fully vested and expected to vest options that are exercisable.

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The number of exercisable share options (fully vested and expected to vest) that may be converted as of the balance sheet date.

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As of the balance sheet date, the weighted-average exercise price (at which grantees can acquire the shares reserved for issuance) for exercisable stock options that are fully vested or expected to vest.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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