Long-Term Incentive Plan (Narrative) (Details) shares in Millions	12 Months Ended
Dec. 31, 2019 shares
Long Term Incentive Plan [Line Items]
Board of Director Deferred Award Days to Payment	60 days
Limited partners' capital account, units issued (in units)	11.9
Limited partner unitholders, units remaining available (in shares)	1.6
Performance Shares [Member]
Long Term Incentive Plan [Line Items]
Equity Based Compensation, Award Payout Percentage, Minimum	0.00%
Equity Based Compensation, Award Payout Percentage, Maximum	200.00%
Equity Based Compensation Award Payout Fluctuation Of Award Market	50.00%
Share-Based Compensation Arrangement By Share-Based Payment Award, Award Forfeiture Period	2 years
Minimum [Member]
Long Term Incentive Plan [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Maximum [Member]
Long Term Incentive Plan [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	4 years

Board of Director Deferred Award Days to Payment

No definition available.

Name:	mmp_BoardofDirectorDeferredAwardDaystoPayment
Namespace Prefix:	mmp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Equity Based Compensation Award Payout Fluctuation Of Award Market

+ References

No definition available.

+ Details

Name:	mmp_EquityBasedCompensationAwardPayoutFluctuationOfAwardMarket
Namespace Prefix:	mmp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Equity Based Compensation, Award Payout Percentage, Maximum

+ References

No definition available.

+ Details

Name:	mmp_EquityBasedCompensationAwardPayoutPercentageMaximum
Namespace Prefix:	mmp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Equity Based Compensation, Award Payout Percentage, Minimum

+ References

No definition available.

+ Details

Name:	mmp_EquityBasedCompensationAwardPayoutPercentageMinimum
Namespace Prefix:	mmp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Long Term Incentive Plan [Line Items]

+ References

No definition available.

+ Details

Name:	mmp_LongTermIncentivePlanLineItems
Namespace Prefix:	mmp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-Based Compensation Arrangement By Share-Based Payment Award, Award Forfeiture Period

+ References

No definition available.

+ Details

Name:	mmp_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardForfeiturePeriod
Namespace Prefix:	mmp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum number of shares authorized for issuance under the deferred compensation arrangement as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

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

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

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

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=116856206&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=us-gaap_PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: