Revenue from Contracts with Customers (Impact of ASU 2014-09 on Balance Sheet and Income Statement) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Insurance Services Revenue	$ 256	$ 241
Nonoperating Income (Expense)	354	396
Revenues	1,403	1,375
Labor and Related Expense	306	285
Other Noninterest Expense	445	405
Noninterest Expense	814	778
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	328	326
Income Tax Expense (Benefit)	76	113
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	252	213
Net Income (Loss) Attributable to Parent	250	214
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	(78)	234
Premiums receivable and other insurance assets	2,197			$ 2,047
Other Assets	6,025			5,663
Assets	170,021	162,101		167,148
Unearned Premiums	2,904			2,604
Liabilities	156,939			153,654
Retained Earnings (Accumulated Deficit)	(6,318)			(6,406)
Stockholders' Equity Attributable to Parent	13,082	13,365	$ 13,375	13,494	$ 13,317
Liabilities and Equity	170,021			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	(6)
Nonoperating Income (Expense)	(6)
Revenues	(6)
Labor and Related Expense	(1)
Other Noninterest Expense	(2)
Noninterest Expense	(3)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(3)
Income Tax Expense (Benefit)	(1)
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(2)
Net Income (Loss) Attributable to Parent	(2)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	(2)
Premiums receivable and other insurance assets	125
Other Assets	42
Assets	167
Unearned Premiums	295
Liabilities	295
Retained Earnings (Accumulated Deficit)	(128)
Stockholders' Equity Attributable to Parent	(128)
Liabilities and Equity	167
Retained Earnings [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net Income (Loss) Attributable to Parent	250	214
Stockholders' Equity Attributable to Parent	$ (6,318)	$ (6,975)	$ (6,510)	$ (6,406)	$ (7,151)