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, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	$ 33,029
Less: payments
Ending liabilities for unpaid losses and loss adjustment expenses, gross	36,188
Group Insurance Policy [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	8,445	$ 8,512
Reinsurance and other recoverables	231	241	$ 239	$ 209
Beginning liabilities for unpaid losses and loss adjustment expenses, net	8,206	8,303
Provision for unpaid losses and loss adjustment expense
Current incurral year	3,351	3,423
Prior year's discount accretion	169	175
Prior incurral year development	(321)	(284)
Total provision for unpaid losses and loss adjustment expenses	3,199	3,314
Less: payments
Current incurral year	(1,603)	(1,659)
Prior incurral years	(1,743)	(1,741)
Total payments	(3,346)	(3,400)
Ending liabilities for unpaid losses and loss adjustment expenses, net	8,059	8,259
Ending liabilities for unpaid losses and loss adjustment expenses, gross	8,290	8,500
Group Insurance Policy [Member] | Other Operating Income (Expense) [Member]
Provision for unpaid losses and loss adjustment expense
Total provision for unpaid losses and loss adjustment expenses	130	131
Aetna Group Insurance [Member] | Group Insurance Policy [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Business Acquisitions	$ 0	$ 42