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Stock-Based Compensation - Schedule of Options Outstanding (Detail) (CAD)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of options outstanding, Beginning balance	4,758,108us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	6,623,242us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	6,867,367us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Number of options, Granted	751,300us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod	1,060,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod	1,389,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
Number of options, Exercised	(1,324,580)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised	(2,805,969)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised	(1,525,159)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
Number of options, Cancelled	(27,499)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod	(119,165)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod	(107,966)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod
Number of options, Vested	0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber	0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber	0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber
Number of options outstanding, Ending balance	4,157,329us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	4,758,108us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	6,623,242us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Weighted average exercise price outstanding, Beginning balance	41.82us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	35.39us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	31.54us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Weighted average exercise price, Granted	106.71us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice	57.02us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice	48.22us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
Weighted average exercise price, Exercised	39.83us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	31.99us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	28.46us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
Weighted average exercise price, Cancelled	60.46us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice	51.46us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice	53.14us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice
Weighted average exercise price, Vested	0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice	0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice	0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice
Weighted average exercise price outstanding, Ending balance	54.05us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	41.82us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	35.39us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Number of options exercisable outstanding, Beginning balance	2,847,109us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	3,227,574us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	2,066,700us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
Number of options exercisable, Exercised	(1,324,580)mga_StockIssuedDuringPeriodExercisableSharesStockOptionsExercised	(2,805,969)mga_StockIssuedDuringPeriodExercisableSharesStockOptionsExercised	(1,525,159)mga_StockIssuedDuringPeriodExercisableSharesStockOptionsExercised
Number of options exercisable, Cancelled	(6,000)mga_ShareBasedCompensationArrangementByShareBasedPaymentAwardExercisableOptionsForfeituresInPeriod	(31,667)mga_ShareBasedCompensationArrangementByShareBasedPaymentAwardExercisableOptionsForfeituresInPeriod	(58,967)mga_ShareBasedCompensationArrangementByShareBasedPaymentAwardExercisableOptionsForfeituresInPeriod
Number of options exercisable, Vested	790,715us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber	2,457,171us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber	2,745,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber
Number of options exercisable outstanding, Ending balance	2,307,244us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	2,847,109us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	3,227,574us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber

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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 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.

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Number of options outstanding, including both vested and non-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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