Revenue from Contracts with Customers (Impact of ASU 2014-09 on Balance Sheet and Income Statement) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Insurance Services Revenue	$ 239	$ 227	$ 495	$ 468
Nonoperating Income (Expense)	364	388	718	784
Revenues	1,458	1,455	2,861	2,830
Labor and Related Expense	292	265	598	550
Other Noninterest Expense	446	420	891	825
Noninterest Expense	839	810	1,653	1,588
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	461	376	789	702
Income Tax Expense (Benefit)	113	122	189	235
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	348	254	600	467
Net Income (Loss) Attributable to Parent	349	252	599	466
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	279	328	201	562
Premiums receivable and other insurance assets	2,247		2,247			$ 2,047
Other Assets	6,014		6,014			5,663
Assets	171,345	164,345	171,345	164,345		167,148
Unearned Premiums	2,957		2,957			2,604
Liabilities	158,206		158,206			153,654
Retained Earnings (Accumulated Deficit)	(6,026)		(6,026)			(6,406)
Stockholders' Equity Attributable to Parent	13,139	13,473	13,139	13,473	$ 13,375	13,494	$ 13,317
Liabilities and Equity	171,345		171,345			167,148
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Insurance Services Revenue	(9)		(15)
Nonoperating Income (Expense)	(9)		(15)
Revenues	(9)		(15)
Labor and Related Expense	(1)		(2)
Other Noninterest Expense	(3)		(5)
Noninterest Expense	(4)		(7)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(5)		(8)
Income Tax Expense (Benefit)	(1)		(2)
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(4)		(6)
Net Income (Loss) Attributable to Parent	(4)		(6)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	(4)		(6)
Premiums receivable and other insurance assets	128		128
Other Assets	43		43
Assets	171		171
Unearned Premiums	304		304
Liabilities	304		304
Retained Earnings (Accumulated Deficit)	(133)		(133)
Stockholders' Equity Attributable to Parent	(133)		(133)
Liabilities and Equity	171		171
Retained Earnings [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net Income (Loss) Attributable to Parent	349	252	599	466
Stockholders' Equity Attributable to Parent	$ (6,026)	$ (6,760)	$ (6,026)	$ (6,760)	$ (6,510)	$ (6,406)	$ (7,151)