Reserve for Unpaid Losses and Loss Adjustment Expenses - GB Liabilities for Unpaid Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	$ 32,287
Less: payments
Ending liabilities for unpaid losses and loss adjustment expenses, gross	32,297
Group Insurance Policies [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	8,512	$ 5,772
Reinsurance recoverables	209	208
Beginning liabilities for unpaid losses and loss adjustment expenses, net	8,303	5,564
Provision for unpaid losses and loss adjustment expense
Current incurral year	3,423	1,960
Prior year's discount accretion	175	148
Prior incurral year development	(284)	(162)
Total provision for unpaid losses and loss adjustment expenses	3,314	1,946
Less: payments
Current incurral year	1,659	917
Prior incurral years	1,741	1,118
Total payments	3,400	2,035
Ending liabilities for unpaid losses and loss adjustment expenses, net	8,259	5,475
Reinsurance recoverables	241	208
Ending liabilities for unpaid losses and loss adjustment expenses, gross	8,500	5,683
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	131	74
Aetna Group Insurance [Member] | Group Insurance Policies [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Business Acquisitions	$ 42	$ 0