Inventories - Narrative (Details) - USD ($) $ in Millions	Dec. 29, 2018	Dec. 30, 2017
Inventory Disclosure [Abstract]
LIFO inventory amount	$ 18.8	$ 26.5
FIFO cost of inventories	91.8	111.0
Average cost basis inventories	53.6	43.9
FIFO value of inventory consigned to others	$ 5.1	$ 4.4

Carrying amount as of the balance sheet date of FIFO value inventory owned by the entity but in the hands of a customer, typically a reseller.

No definition available.

+ Details

Name:	mli_FifoValueOfInventoryConsignedToOthers
Namespace Prefix:	mli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of FIFO (first in first out) inventory present at the reporting date when inventory is also valued using different valuation methods.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6361739&loc=d3e7789-107766

+ Details

Name:	us-gaap_FIFOInventoryAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

The amount of LIFO (last in first out) inventory present at the reporting date when inventory is also valued using different valuation methods.

+ References

+ Details

Name:	us-gaap_LIFOInventoryAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of weighted average cost inventory present at the reporting date when inventory is valued using different valuation methods.

+ References

+ Details

Name:	us-gaap_WeightedAverageCostInventoryAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant