The percentage of total net revenues generated by AFSC.

No definition available.

Name:	mgln_PercentageOfTotalRevenuesAttributableToAfsc
Namespace Prefix:	mgln_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of shares repurchased by the entity pursuant to the share repurchase program.

+ References

No definition available.

+ Details

Name:	mgln_SharesRepurchasedOpenMarketPurchasesMadeShares
Namespace Prefix:	mgln_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate cost of stock that was repurchased during the period, excluding broker commissions.

+ References

No definition available.

+ Details

Name:	mgln_SharesRepurchasedValueExcludingBrokerCommissions
Namespace Prefix:	mgln_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The increase in amount authorized by an entity's board of directors under a stock repurchase plan.

+ References

No definition available.

+ Details

Name:	mgln_StockRepurchaseProgramAuthorizedAmountIncreaseDecrease
Namespace Prefix:	mgln_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Amount of stock repurchase plan authorized.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramAuthorizedAmount1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount remaining of a stock repurchase plan authorized.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Total cost of shares repurchased divided by the total number of shares repurchased.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

+ Details

Name:	us-gaap_TreasuryStockAcquiredAverageCostPerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ShareRepurchaseProgramAxis=mgln_ShareRepurchaseProgramOctober2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended		36 Months Ended
	May 24, 2018	Oct. 31, 2018	Dec. 31, 2015	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Oct. 26, 2015
Stock Repurchases
Percentage of total net revenues generated by AFSC				2.00%	3.00%
October 2015 Share Repurchase Program
Stock Repurchases
Amount authorized under stock repurchase plan	$ 400.0							$ 200.0
Increase in amount authorized under stock repurchase plan	$ 200.0
Share repurchases made in open market (in shares)		165,039	345,044	514,669	280,140	1,828,183	2,968,036
Average Price Paid per Share (in dollars per share)			$ 53.46	$ 81.67	$ 77.67	$ 58.40
Aggregate Cost		$ 11.2	$ 18.4	$ 42.0	$ 21.8	$ 106.8	$ 189.0
Remaining authorized repurchase amount				$ 211.0			$ 211.0