Partners' Capital and Distributions (Narrative) (Details) $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($)
Partners' Capital [Abstract]
Amount Authorized Under Equity Distribution Agreement	$ 750.0

Amount Authorized Under Equity Distribution Agreement

No definition available.

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