Pension and Other Postretirement Benefits - About our Plans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Employees	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Cigna Pension Plan And Cigna Pension Plan For Certain Former Employees [Member]
Defined Benefit Plan Disclosure [Line Items]
Covered retirees | Employees	22,200
Covered vested former employees | Employees	14,500
Covered active employees | Employees	14,000
Tax status	us-gaap:QualifiedPlanMember
Funding status	us-gaap:FundedPlanMember
Sponsor location	country:US
Plan Type	us-gaap:PensionPlansDefinedBenefitMember
Cigna Supplemental Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Tax status	us-gaap:NonqualifiedPlanMember
Funding status	us-gaap:UnfundedPlanMember
Sponsor location	country:US
Plan Type	us-gaap:PensionPlansDefinedBenefitMember
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent at which excess actuarial gains (losses) are amortized to other operating expense	10.00%
Period for recognizing differences between actual and expected long-term returns to determine penison costs	5 years
United States [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Market-related valuation of pension plan assets	$ 4,100
Plan assets at fair value	4,281	$ 3,977	$ 3,981
Defined benefit plan cost	2	18	(1)
Defined benefit plan benefit obligation	4,969	4,888	4,934
United States [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	2	5	8
Defined benefit plan cost	7	9	8
Defined benefit plan benefit obligation	$ 258	$ 277	$ 295
United States [Member] | Postretirement medical plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Covered retirees | Employees	18,400
Covered active employees | Employees	18,600
Percentage of year 2000 per capita cost at which defined benefit plan cost is capped for post-1988 retirees	200.00%
Funding status	us-gaap:UnfundedPlanMember
Foreign Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value
Defined benefit plan cost
Defined benefit plan benefit obligation