Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Supplementary Insurance Information, by Segment [Line Items]
Reduction of operating expenses resulting from tax reform	$ (2)	$ 0	$ (2)	$ 0

(Decrease) to operating expenses for measurement period adjustment as a result of the Tax Cuts and Jobs Act of 2017 reflecting a decrease in a liability to reimburse a reinsurer for taxes related to a block of business sold through reinsurance.

No definition available.

Name:	ci_TaxCutsAndJobsActOf2017MeasurementPeriodAdjustmentChangeInTaxRateOperatingExpenseBenefit
Namespace Prefix:	ci_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration