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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	$ 28,317
Less: payments
Ending liabilities for unpaid losses and loss adjustment expenses, gross	32,287	$ 28,317
Reinsurance recoverables, net				$ 4,061	$ 3,659
Group Insurance Policies
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	5,772	5,889	$ 6,013
Reinsurance and other recoverables	208	218	209
Beginning liabilities for unpaid losses and loss adjustment expenses, net	8,303	5,564	5,671	8,303	5,564	$ 5,671	$ 5,804
Add: Aetna U.S. group life and disability business acquisitions	2,833	0	0
Provision for unpaid losses and loss adjustment expenses
Current accident year	2,868	2,562	2,447
Prior year's discount accretion	202	202	214
Prior accident year development	(185)	(162)	(146)
Total provision for unpaid losses and loss adjustment expenses	2,885	2,602	2,515
Less: payments
Current incurral year	1,528	1,327	1,257
Prior incurral years	1,451	1,382	1,391
Total payments	2,979	2,709	2,648
Ending liabilities for unpaid losses and loss adjustment expenses, net	8,303	5,564	5,671
Reinsurance and other recoverables	209	208	218
Ending liabilities for unpaid losses and loss adjustment expenses, gross	$ 8,512	$ 5,772	$ 5,889
Reinsurance recoverables, net				$ 111	$ 100	$ 96