v3.8.0.1

Short-term and Long-term Investments, Restricted Deposits - Schedule by investment Type (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | $ 5,982 | $ 5,233
Gross Unrealized Gains | 27 | 15
Gross Unrealized Losses | (31) | (60)
Fair Value | 5,978 | 5,188
U.S. Treasury securities and obligations of U.S. government corporations and agencies |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 311 | 364
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (2) | (1)
Fair Value | 309 | 363
Corporate securities |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 2,208 | 1,933
Gross Unrealized Gains | 12 | 12
Gross Unrealized Losses | (10) | (13)
Fair Value | 2,210 | 1,932
Restricted certificates of deposit |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 4 | 5
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 4 | 5
Restricted cash equivalents |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 17 | 6
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 17 | 6
Municipal securities |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 2,085 | 1,767
Gross Unrealized Gains | 12 | 1
Gross Unrealized Losses | (10) | (35)
Fair Value | 2,087 | 1,733
Asset-backed securities |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 437 | 317
Gross Unrealized Gains | 1 | 1
Gross Unrealized Losses | (1) | (1)
Fair Value | 437 | 317
Residential mortgage-backed securities |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 337 | 219
Gross Unrealized Gains | 1 | 1
Gross Unrealized Losses | (6) | (5)
Fair Value | 332 | 215
Commercial mortgage- backed securities |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 272 | 343
Gross Unrealized Gains | 1 | 0
Gross Unrealized Losses | (2) | (5)
Fair Value | 271 | 338
Equity method investments |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 176 | 163
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 176 | 163
Life insurance contracts |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 135 | 116
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | $ 135 | $ 116

X
- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
Name: | us-gaap_AvailableForSaleSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + 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 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=cnc_RestrictedCertificatesOfDepositMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=cnc_RestrictedCashEquivalentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MunicipalBondsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=cnc_CostMethodInvestmentsAndEquityMethodSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=cnc_LifeInsuranceContractsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |