Revenue from Contracts with Customers (Contract with Customer, Asset and Liability) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Premiums receivable and other insurance assets	$ 2,247		$ 2,047
Other Assets	6,014		5,663
Assets	171,345		167,148	$ 164,345
Unearned Premiums	2,957		2,604
Liabilities	158,206		153,654
Retained Earnings (Accumulated Deficit)	(6,026)		(6,406)
Stockholders' Equity Attributable to Parent	13,139	$ 13,375	13,494	$ 13,473	$ 13,317
Liabilities and Equity	$ 171,345		$ 167,148
Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Premiums receivable and other insurance assets		122
Other Assets		41
Assets		163
Unearned Premiums		289
Liabilities		289
Retained Earnings (Accumulated Deficit)		(126)
Stockholders' Equity Attributable to Parent		(126)
Liabilities and Equity		163
Day One Balance [Member]
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 Premiums		2,893
Liabilities		153,943
Retained Earnings (Accumulated Deficit)		(6,532)
Stockholders' Equity Attributable to Parent		13,368
Liabilities and Equity		$ 167,311