Capital Stock (Stock Options Activity) (Details) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
Mar. 31, 2021 USD ($) $ / shares shares
Stock Options and SARs \| Class A Voting Common Shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	7.2
Granted (in shares)	0.0
Exercised (in shares)	0.0
Awards canceled in exchange program (in shares)	(1.1)
Awards issued in exchange program (in shares)	0.1
Forfeited or expired (in shares)	(0.7)
Outstanding, end of period (in shares)	5.5
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding, Beginning of Period, Weighted Average Exercise Price (in usd per share) \| $ / shares	$ 26.21
Granted, Weighted Average Exercise Price (in usd per share) \| $ / shares	0
Exercised, Weighted Average Exercise Price (in usd per share) \| $ / shares	6.20
Awards canceled in exchange program, weighted average exercise price (in usd per share) \| $ / shares	31.56
Awards issued in exchange program, Weighted Average Exercise Price (in usd per share) \| $ / shares	7.70
Forfeited or expired, Weighted Average Exercise Price (in usd per share) \| $ / shares	31.28
Outstanding, End of Period, Weighted Average Exercise Price (in usd per share) \| $ / shares	$ 24.23
Outstanding, Weighted Average Remaining Contractual Term (in years)	3 years 7 months 2 days
Outstanding, Aggregate Intrinsic Value \| $	$ 0.8
Outstanding, vested or expected to vest (in shares)	5.5
Outstanding, vested or expected to vest, Weighted Average Exercise Price (in usd per share) \| $ / shares	$ 24.25
Outstanding, vested or expected to vest, Weighted Average Remaining Contractual Term In Years	3 years 7 months 2 days
Outstanding, vested or expected to vest, Aggregate Intrinsic Value \| $	$ 0.7
Exercisable (in shares)	4.4
Exercisable, Weighted Average Exercise Price (in usd per share) \| $ / shares	$ 25.07
Exercisable, Weighted Average Remaining Contractual Term In Years	3 years 3 months 18 days
Exercisable, Aggregate Intrinsic Value \| $	$ 0.0
Stock Options and SARs \| Class B Non-Voting Common Shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	28.5
Granted (in shares)	5.4
Exercised (in shares)	(0.4)
Awards canceled in exchange program (in shares)	(4.3)
Awards issued in exchange program (in shares)	0.8
Forfeited or expired (in shares)	(8.8)
Outstanding, end of period (in shares)	21.2
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding, Beginning of Period, Weighted Average Exercise Price (in usd per share) \| $ / shares	$ 19.03
Granted, Weighted Average Exercise Price (in usd per share) \| $ / shares	8.74
Exercised, Weighted Average Exercise Price (in usd per share) \| $ / shares	8.96
Awards canceled in exchange program, weighted average exercise price (in usd per share) \| $ / shares	26.49
Awards issued in exchange program, Weighted Average Exercise Price (in usd per share) \| $ / shares	7.13
Forfeited or expired, Weighted Average Exercise Price (in usd per share) \| $ / shares	16.49
Outstanding, End of Period, Weighted Average Exercise Price (in usd per share) \| $ / shares	$ 15.85
Outstanding, Weighted Average Remaining Contractual Term (in years)	6 years 5 months 12 days
Outstanding, Aggregate Intrinsic Value \| $	$ 29.8
Outstanding, vested or expected to vest (in shares)	21.0
Outstanding, vested or expected to vest, Weighted Average Exercise Price (in usd per share) \| $ / shares	$ 15.93
Outstanding, vested or expected to vest, Weighted Average Remaining Contractual Term In Years	6 years 5 months 1 day
Outstanding, vested or expected to vest, Aggregate Intrinsic Value \| $	$ 28.9
Exercisable (in shares)	11.0
Exercisable, Weighted Average Exercise Price (in usd per share) \| $ / shares	$ 20.39
Exercisable, Weighted Average Remaining Contractual Term In Years	4 years 7 months 28 days
Exercisable, Aggregate Intrinsic Value \| $	$ 2.0
SARs \| Class B Non-Voting Common Shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)	3.8
Maximum \| Stock Options and SARs \| Class B Non-Voting Common Shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Exercised (in shares)	(0.1)

Number of share options (or share units) canceled in exchange program during the period.

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Weighted average price of options that were canceled in exchange program during the period.

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Number of share options (or share units) issued in exchange program during the period.

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Weighted average exercise price of options that were issued in exchange program during the period.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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

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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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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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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of share options (or share units) exercised during the current period.

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