Segment Information - Special Items (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Before-tax [Abstract]
Transaction-related costs	$ 130	$ 16	$ 190	$ 79
Long-term care guaranty fund assessment			0	129
Total impact from special items, before-tax	130	16	190	208
After-tax [Abstract]
Transaction-related costs	109	(47)	159	2
Long-term care guaranty fund assessment			0	83
Total impact of special items, after-tax	$ 109	$ (47)	$ 159	$ 85

Before-tax charges (treated as a special item) related to the liquidation of Penn Treaty Network America Insurance Company, together with its subsidiary American Network Insurance Company (collectively 'Penn Treaty', a long-term care insurance carrier), triggering guaranty fund coverage and accrual of a liability. Special items are excluded from the calculation of 'adjusted income (loss) from operations' because management believes they are not representative of the underlying results of operations. This is generally because the nature and size of these matters are not indicative of our ongoing business operations.

No definition available.

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