Parent and Guarantor Condensed Consolidating Financial Statements (Schedule of Condensed Consolidating Balance Sheet) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and Due from Banks	$ 768		$ 844
Interest-bearing Deposits in Banks and Other Financial Institutions	2,953		3,408
Cash and Cash Equivalents, at Carrying Value	3,721		4,252	$ 4,302
Equity Securities	680		518
Available-for-sale securities	22,726		22,303
Held-to-maturity securities	1,967		1,899
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	126		108
Loans and Leases Receivable, Gross	125,327		122,893	119,002
Loans to nonbank subsidiary	0		0
Financing Receivable, Allowance for Credit Losses	1,278		1,276	1,155	$ 1,144
Loans and Leases Receivable, Net Amount	124,049		121,617
Property Subject to or Available for Operating Lease, Net	8,530		8,741
Receivables from bank subsidiary	0		0
Receivables from nonbank subsidiary	0		0
Investment in bank subsidiaries	0		0
Investment in nonbank subsidiaries	0		0
Premiums receivable and other insurance assets	2,197		2,047
Other Assets	6,025		5,663
Assets	170,021		167,148	162,101
Noninterest-bearing deposit liabilities	122		108
Interest-bearing Deposit Liabilities	97,324		93,148
Deposits	97,446		93,256
Short-term Debt	9,564		11,413
Long-term Debt	45,076		44,226
Debt Payable To Bank Subsidiaries	0		0
Debt Payable To Nonbank Subsidiaries	0		0
Payables to Bank Subsidiaries			0
Payables to Nonbank Subsidiaries	0		0
Interest payable	494		375
Unearned Premiums	2,904		2,604
Accounts Payable and Accrued Liabilities	1,455		1,780
Liabilities	156,939		153,654
Stockholders' Equity Attributable to Parent	13,082	$ 13,375	13,494	13,365	$ 13,317
Liabilities and Equity	170,021		167,148
Parent Company [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and Due from Banks	56		74
Interest-bearing Deposits in Banks and Other Financial Institutions	5		5
Cash and Cash Equivalents, at Carrying Value	796		1,217	1,490
Equity Securities	0		0
Available-for-sale securities	0		0
Held-to-maturity securities	0		0
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	0		0
Loans and Leases Receivable, Gross	6,443		7,434
Loans to nonbank subsidiary	1,311		879
Financing Receivable, Allowance for Credit Losses	145		185
Loans and Leases Receivable, Net Amount	7,609		8,128
Property Subject to or Available for Operating Lease, Net	14		19
Receivables from bank subsidiary	86		80
Receivables from nonbank subsidiary	41		71
Investment in bank subsidiaries	16,163		16,962
Investment in nonbank subsidiaries	7,520		8,111
Premiums receivable and other insurance assets	0		0
Other Assets	2,221		2,207
Assets	34,450		36,795
Noninterest-bearing deposit liabilities	0		0
Interest-bearing Deposit Liabilities	6		12
Deposits	6		12
Short-term Debt	2,957		3,171
Long-term Debt	16,808		17,966
Debt Payable To Bank Subsidiaries	65		74
Debt Payable To Nonbank Subsidiaries	992		1,103
Payables to Bank Subsidiaries			4
Payables to Nonbank Subsidiaries	115		132
Interest payable	209		200
Unearned Premiums	0		0
Accounts Payable and Accrued Liabilities	216		639
Liabilities	21,368		23,301
Stockholders' Equity Attributable to Parent	13,082		13,494
Liabilities and Equity	34,450		36,795
Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and Due from Banks	0		0
Interest-bearing Deposits in Banks and Other Financial Institutions	0		0
Cash and Cash Equivalents, at Carrying Value	0		0	0
Equity Securities	0		0
Available-for-sale securities	0		0
Held-to-maturity securities	0		0
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	0		0
Loans and Leases Receivable, Gross	0		0
Loans to nonbank subsidiary	0		0
Financing Receivable, Allowance for Credit Losses	0		0
Loans and Leases Receivable, Net Amount	0		0
Property Subject to or Available for Operating Lease, Net	0		0
Receivables from bank subsidiary	0		0
Receivables from nonbank subsidiary	0		0
Investment in bank subsidiaries	16,163		16,962
Investment in nonbank subsidiaries	0		0
Premiums receivable and other insurance assets	0		0
Other Assets	0		0
Assets	16,163		16,962
Noninterest-bearing deposit liabilities	0		0
Interest-bearing Deposit Liabilities	0		0
Deposits	0		0
Short-term Debt	0		0
Long-term Debt	0		0
Debt Payable To Bank Subsidiaries	0		0
Debt Payable To Nonbank Subsidiaries	0		0
Payables to Bank Subsidiaries			0
Payables to Nonbank Subsidiaries	0		0
Interest payable	0		0
Unearned Premiums	0		0
Accounts Payable and Accrued Liabilities	0		0
Liabilities	0		0
Stockholders' Equity Attributable to Parent	16,163		16,962
Liabilities and Equity	16,163		16,962
Non-Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and Due from Banks	712		770
Interest-bearing Deposits in Banks and Other Financial Institutions	2,948		3,403
Cash and Cash Equivalents, at Carrying Value	4,237		4,868	3,453
Equity Securities	680		518
Available-for-sale securities	22,726		22,303
Held-to-maturity securities	2,033		1,973
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	126		108
Loans and Leases Receivable, Gross	118,884		115,459
Loans to nonbank subsidiary	415		408
Financing Receivable, Allowance for Credit Losses	1,133		1,091
Loans and Leases Receivable, Net Amount	118,166		114,776
Property Subject to or Available for Operating Lease, Net	8,516		8,722
Receivables from bank subsidiary	0		0
Receivables from nonbank subsidiary	61		77
Investment in bank subsidiaries	0		0
Investment in nonbank subsidiaries	0		0
Premiums receivable and other insurance assets	2,197		2,082
Other Assets	5,072		5,105
Assets	163,814		160,532
Noninterest-bearing deposit liabilities	122		108
Interest-bearing Deposit Liabilities	97,318		93,136
Deposits	98,175		94,383
Short-term Debt	6,607		8,242
Long-term Debt	28,268		26,260
Debt Payable To Bank Subsidiaries	0		0
Debt Payable To Nonbank Subsidiaries	1,311		879
Payables to Bank Subsidiaries			0
Payables to Nonbank Subsidiaries	106		127
Interest payable	285		175
Unearned Premiums	2,904		2,604
Accounts Payable and Accrued Liabilities	2,473		2,790
Liabilities	140,129		135,460
Stockholders' Equity Attributable to Parent	23,685		25,072
Liabilities and Equity	163,814		160,532
Affiliated Entity [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Interest-bearing Deposits in Banks and Other Financial Institutions	0		0
Interest-bearing Deposit Liabilities	0		0
Affiliated Entity [Member] | Parent Company [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Interest-bearing Deposits in Banks and Other Financial Institutions	735		1,138
Interest-bearing Deposit Liabilities	0		0
Affiliated Entity [Member] | Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Interest-bearing Deposits in Banks and Other Financial Institutions	0		0
Interest-bearing Deposit Liabilities	0		0
Affiliated Entity [Member] | Non-Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Interest-bearing Deposits in Banks and Other Financial Institutions	577		695
Interest-bearing Deposit Liabilities	735		1,139
Consolidation, Eliminations [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and Due from Banks	0		0
Interest-bearing Deposits in Banks and Other Financial Institutions	0		0
Cash and Cash Equivalents, at Carrying Value	(1,312)		(1,833)	$ (641)
Equity Securities	0		0
Available-for-sale securities	0		0
Held-to-maturity securities	(66)		(74)
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	0		0
Loans and Leases Receivable, Gross	0		0
Loans to nonbank subsidiary	(1,726)		(1,287)
Financing Receivable, Allowance for Credit Losses	0		0
Loans and Leases Receivable, Net Amount	(1,726)		(1,287)
Property Subject to or Available for Operating Lease, Net	0		0
Receivables from bank subsidiary	(86)		(80)
Receivables from nonbank subsidiary	(102)		(148)
Investment in bank subsidiaries	(32,326)		(33,924)
Investment in nonbank subsidiaries	(7,520)		(8,111)
Premiums receivable and other insurance assets	0		(35)
Other Assets	(1,268)		(1,649)
Assets	(44,406)		(47,141)
Noninterest-bearing deposit liabilities	0		0
Interest-bearing Deposit Liabilities	0		0
Deposits	(735)		(1,139)
Short-term Debt	0		0
Long-term Debt	0		0
Debt Payable To Bank Subsidiaries	(65)		(74)
Debt Payable To Nonbank Subsidiaries	(2,303)		(1,982)
Payables to Bank Subsidiaries			(4)
Payables to Nonbank Subsidiaries	(221)		(259)
Interest payable	0		0
Unearned Premiums	0		0
Accounts Payable and Accrued Liabilities	(1,234)		(1,649)
Liabilities	(4,558)		(5,107)
Stockholders' Equity Attributable to Parent	(39,848)		(42,034)
Liabilities and Equity	(44,406)		(47,141)
Consolidation, Eliminations [Member] | Affiliated Entity [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Interest-bearing Deposits in Banks and Other Financial Institutions	(1,312)		(1,833)
Interest-bearing Deposit Liabilities	$ (735)		$ (1,139)