Stock-Based Compensation - Additional Information (Detail)	12 Months Ended
Dec. 31, 2014 Incentive_Stock_Option_Plan	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of incentive stock option plans	2mga_NumberOfIncentiveStockOptionPlans
Holding period of restricted stock available to executives	4 years
Share based compensation arrangement long-term retention program restricted stock available to executive releasing period	10 years
Number of days considered for weighted average trading price	20 days
Percentage of annual retainers	60.00%
Percentage of additional annual retainers	100.00%
Stock Option Plan 1987 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period of granted option		7 years
Stock Option Plan 2009 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares, issued under the option plan	16,000,000
Number of shares available to be granted	6,793,030	7,516,831
Period of granted option	7 years
Option vested under stock option plan to employees and consultants	One-third on each of the first three anniversaries of the date of grant.

Number of days considered for weighted average trading price.

No definition available.

Name:	mga_NumberOfDaysConsideredForWeightedAverageTradingPrice
Namespace Prefix:	mga_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number Of Incentive Stock Option Plans

+ References

No definition available.

+ Details

Name:	mga_NumberOfIncentiveStockOptionPlans
Namespace Prefix:	mga_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of annual retainers deferment for director share based compensation plan maximum.

+ References

No definition available.

+ Details

Name:	mga_PercentageOfAnnualRetainersDefermentForDirectorShareBasedCompensationPlanMaximum
Namespace Prefix:	mga_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of annual retainers for director share based compensation plan.

+ References

No definition available.

+ Details

Name:	mga_PercentageOfAnnualRetainersForDirectorShareBasedCompensationPlan
Namespace Prefix:	mga_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share based compensation arrangement long term retention program restricted stock available to executive holding period.

+ References

No definition available.

+ Details

Name:	mga_ShareBasedCompensationArrangementLongTermRetentionProgramRestrictedStockAvailableToExecutiveHoldingPeriod
Namespace Prefix:	mga_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Share based compensation arrangement long term retention program restricted stock available to executive releasing period.

+ References

No definition available.

+ Details

Name:	mga_ShareBasedCompensationArrangementLongTermRetentionProgramRestrictedStockAvailableToExecutiveReleasingPeriod
Namespace Prefix:	mga_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of award terms as to how many shares or portion of an award are no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, to sell the shares, and be entitled to the cash proceeds of such sale. For example, vesting may be expressed as being 25 percent of the shares under option on each anniversary of the grant date.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRights
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_AwardTypeAxis=mga_StockOptionPlanOneThousandNineHundredEightySevenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=mga_StockOptionPlanTwoThousandNineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: