Investments - Gross Unrealized Appreciation (Depreciation) on Fixed Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 22,869	$ 21,867
Unrealized Appreciation	826	1,371
Unrealized (Depreciation)	(427)	(100)
Total Fair Value	23,268	23,138
Run-off Settlement Annuity Business [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,266	2,200
Unrealized Appreciation	471	681
Unrealized (Depreciation)	(27)	(2)
Total Fair Value	2,710	2,879
Federal government and agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	558	541
Unrealized Appreciation	194	239
Unrealized (Depreciation)	(5)	(1)
Total Fair Value	747	779
State and local government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	934	1,196
Unrealized Appreciation	59	93
Unrealized (Depreciation)	(3)	(2)
Total Fair Value	990	1,287
Foreign government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,369	2,360
Unrealized Appreciation	136	142
Unrealized (Depreciation)	(14)	(15)
Total Fair Value	2,491	2,487
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,476	17,301
Unrealized Appreciation	421	868
Unrealized (Depreciation)	(395)	(81)
Total Fair Value	18,502	18,088
Mortgage and other asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	532	469
Unrealized Appreciation	16	29
Unrealized (Depreciation)	(10)	(1)
Total Fair Value	$ 538	$ 497

CIGNA CORP 10-Q