Reserve for Unpaid Losses and Loss Adjustment Expenses - Group Benefits Liabilities for Unpaid Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	$ 33,029
Payments
Ending liabilities for unpaid losses and loss adjustment expenses, gross	36,517	$ 33,029
Group Benefits
Liability for Claims and Claims Adjustment Expense [Line Items]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	8,445	8,512	$ 5,772
Reinsurance and other recoverables	239	209	208
Beginning liabilities for unpaid losses and loss adjustment expenses, net	8,206	8,303	5,564
Provision for unpaid losses and loss adjustment expenses
Current accident year	4,385	4,470	2,868
Prior year's discount accretion	219	227	202
Prior accident year development [1]	(410)	(324)	(185)
Total provision for unpaid losses and loss adjustment expenses	4,194	4,373	2,885
Payments
Current incurral year	(2,277)	(2,377)	(1,528)
Prior incurral years	(2,114)	(2,135)	(1,451)
Total payments	(4,391)	(4,512)	(2,979)
Ending liabilities for unpaid losses and loss adjustment expenses, net	8,009	8,206	8,303
Reinsurance and other recoverables	247	239	209
Ending liabilities for unpaid losses and loss adjustment expenses, gross	8,256	8,445	8,512
Group Benefits | Aetna Group Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Add: Aetna U.S. group life and disability business acquisitions	0	42	2,833
Other Operating Income (Expense) [Member] | Group Benefits
Provision for unpaid losses and loss adjustment expenses
Total provision for unpaid losses and loss adjustment expenses	$ 178	$ 194	$ 111