Revenue from Contracts with Customers (Contract with Customer, Asset and Liability) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Premiums receivable and other insurance assets	$ 2,326		$ 2,047
Other assets	6,153		5,663
Assets	178,869		167,148	$ 163,728
Unearned insurance premiums and service revenue	3,044		2,604
Liabilities	165,601		153,654
Accumulated deficit	(5,489)		(6,406)
Equity	13,268	$ 13,375	13,494	$ 13,317	$ 13,439
Liabilities and equity	$ 178,869		$ 167,148
Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Premiums receivable and other insurance assets		122
Other assets		41
Assets		163
Unearned insurance premiums and service revenue		289
Liabilities		289
Accumulated deficit		(126)
Equity		(126)
Liabilities and equity		163
Day one balance
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Premiums receivable and other insurance assets		2,169
Other assets		5,704
Assets		167,311
Unearned insurance premiums and service revenue		2,893
Liabilities		153,943
Accumulated deficit		(6,532)
Equity		13,368
Liabilities and equity		$ 167,311