Fair Value Measurements - Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair values of long-term debt	$ 1,255	$ 1,260
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair values of long-term debt	$ 1,347	1,707
5.375% Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate	5.375%
4.875% Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate	4.875%
1.125% Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate	1.125%
Senior Notes \| 5.375% Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate	5.375%
Senior Notes \| 5.375% Notes \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair values of long-term debt	$ 696	694
Senior Notes \| 5.375% Notes \| Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair values of long-term debt	$ 745	674
Senior Notes \| 4.875% Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate	4.875%
Senior Notes \| 4.875% Notes \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair values of long-term debt	$ 327	326
Senior Notes \| 4.875% Notes \| Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair values of long-term debt	$ 340	301
Senior Notes \| 1.125% Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate	1.125%
Term Loan Facility \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair values of long-term debt	$ 220	0
Term Loan Facility \| Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair values of long-term debt	$ 220	0
1.125% Convertible Notes \| 4.875% Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate	4.875%
1.125% Convertible Notes \| 1.125% Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate	1.125%
1.125% Convertible Notes \| 1.125% Convertible Notes \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair values of long-term debt	$ 12	240
1.125% Convertible Notes \| 1.125% Convertible Notes \| Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair values of long-term debt	$ 42	732
1.125% Conversion Option
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate on derivative	1.125%
1.125% Conversion Option
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
1.125% Conversion Option derivative liability	$ 29	$ 476
Percentage of contractual interest rate on derivative	1.125%

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

+ Details

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

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

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=moh_SeniorNotesDue2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=moh_A4.875SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=moh_CashConvertibleSeniorNotesDue2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_ConvertibleDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_EmbeddedDerivativeFinancialInstrumentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_EmbeddedDerivativeFinancialInstrumentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: