v2.4.0.8

Debt (Narrative) (Details) (USD $)	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 2,900,000,000.0		$ 2,700,000,000.0
Debt placement costs	(2,887,000)	0
Senior Notes [Member] | 5.15% Notes Due 2043 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	550,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	5.15%
Senior Notes [Member] | 5.15% Notes Due 2043 [Member] | Debt Offering Part Two [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	5.15%
Notes At Price	103.10%
After Underwriting Discounts	255,000,000
Debt placement costs	(2,700,000)
Senior Notes [Member] | 6.45% Notes Due 2014 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	6.45%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowing capacity	1,000,000,000.0
Unused commitment fee	0.125%
Obligation for letters of credit	5,600,000
Revolving Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.00%
Unused commitment fee	0.10%
Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.75%
Unused commitment fee	0.28%
Commercial Paper [Member]
Debt Instrument [Line Items]
Borrowing capacity	$ 1,000,000,000.0
Commercial Paper [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Term	397 days

X

- Definition

After Underwriting Discounts

+ References

No definition available.

+ Details

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Namespace Prefix:	mmp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Notes At Price

+ References

No definition available.

+ Details

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Namespace Prefix:	mmp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
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- Definition

The cash outflow paid to third parties in connection with debt origination, which will be amortized over the remaining maturity period of the associated long-term debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 15

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585

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