Reserve for Unpaid Losses and Loss Adjustment Expenses - GB Liabilities for Unpaid Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	$ 36,517
Less: payments
Ending liabilities for unpaid losses and loss adjustment expenses, gross	36,582
Group Insurance Policy [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	8,256	$ 8,445
Reinsurance recoverables [1]	249	237		$ 246	$ 239
Beginning liabilities for unpaid losses and loss adjustment expenses, net	8,010	8,206
Provision for unpaid losses and loss adjustment expense
Current incurral year	1,148	1,150
Prior year's discount accretion	57	58
Prior incurral year development [2]	(163)	(120)
Total provision for unpaid losses and loss adjustment expenses	1,042	1,088
Less: payments
Current incurral year	(278)	(314)
Prior incurral years	(821)	(855)
Total payments	(1,099)	(1,169)
Ending liabilities for unpaid losses and loss adjustment expenses, net	7,953	8,125
Ending liabilities for unpaid losses and loss adjustment expenses, gross	8,202	8,362
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	$ 44	$ 46
Accounting Standards Update 2016-13 [Member]
Less: payments
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			$ (2)
Reinsurance Recoverable Including Reinsurance Premium Paid [Member] | Accounting Standards Update 2016-13 [Member] | Group Insurance Policy [Member]
Less: payments
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			$ (1)