Parent and Guarantor Condensed Consolidating Financial Statements (Schedule of Condensed Consolidating Balance Sheet) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents, noninterest-bearing	$ 802			$ 844
Cash and cash equivalents, interest-bearing	2,970			3,408
Cash and cash equivalents	3,772			4,252	$ 4,424
Equity securities	514			518
Available-for-sale securities	24,122			22,303
Held-to-maturity securities	2,246			1,899
Loans held-for-sale, net	425			108
Finance receivables and loans, net	126,605			122,893	118,871
Intercompany loans to nonbank subsidiary	0			0
Allowance for loan losses	1,248	$ 1,257		1,276	1,286	$ 1,225	$ 1,144
Total finance receivables and loans, net	125,357			121,617
Investment in operating leases, net	8,578			8,741
Intercompany receivables from bank subsidiary	0			0
Intercompany receivables from nonbank subsidiary	0			0
Investment in bank subsidiaries	0			0
Investment in nonbank subsidiaries	0			0
Premiums receivable and other insurance assets	2,291			2,047
Other assets	5,796			5,663
Assets	173,101			167,148	164,013
Deposit liabilities, noninterest-bearing	180			108
Deposit liabilities, interest-bearing	101,199			93,148
Deposit liabilities	101,379			93,256
Short-term borrowings	7,338			11,413
Long-term debt	45,542			44,226
Intercompany debt payable to bank subsidiaries	0			0
Intercompany debt payable to nonbank subsidiaries	0			0
Intercompany payables to bank subsidiary	0			0
Intercompany payables to nonbank subsidiaries	0			0
Interest payable	712			375
Unearned insurance premiums and service revenue	3,020			2,604
Accrued expenses and other liabilities	2,025			1,780
Total liabilities	160,016			153,654
Equity	13,085		$ 13,375	13,494	13,573		$ 13,317
Total liabilities and equity	173,101			167,148
Parent
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents, noninterest-bearing	50			74
Cash and cash equivalents, interest-bearing	5			5
Cash and cash equivalents	968			1,217	1,574
Equity securities	0			0
Available-for-sale securities	0			0
Held-to-maturity securities	0			0
Loans held-for-sale, net	0			0
Finance receivables and loans, net	4,379			7,434
Intercompany loans to nonbank subsidiary	821			879
Allowance for loan losses	98			185
Total finance receivables and loans, net	5,102			8,128
Investment in operating leases, net	7			19
Intercompany receivables from bank subsidiary	113			80
Intercompany receivables from nonbank subsidiary	44			71
Investment in bank subsidiaries	16,057			16,962
Investment in nonbank subsidiaries	6,999			8,111
Premiums receivable and other insurance assets	0			0
Other assets	2,220			2,207
Assets	31,510			36,795
Deposit liabilities, noninterest-bearing	0			0
Deposit liabilities, interest-bearing	3			12
Deposit liabilities	3			12
Short-term borrowings	2,575			3,171
Long-term debt	14,111			17,966
Intercompany debt payable to bank subsidiaries	23			74
Intercompany debt payable to nonbank subsidiaries	974			1,103
Intercompany payables to bank subsidiary	45			4
Intercompany payables to nonbank subsidiaries	117			132
Interest payable	242			200
Unearned insurance premiums and service revenue	0			0
Accrued expenses and other liabilities	335			639
Total liabilities	18,425			23,301
Equity	13,085			13,494
Total liabilities and equity	31,510			36,795
Guarantors
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents, noninterest-bearing	0			0
Cash and cash equivalents, interest-bearing	0			0
Cash and cash equivalents	0			0	0
Equity securities	0			0
Available-for-sale securities	0			0
Held-to-maturity securities	0			0
Loans held-for-sale, net	0			0
Finance receivables and loans, net	0			0
Intercompany loans to nonbank subsidiary	0			0
Allowance for loan losses	0			0
Total finance receivables and loans, net	0			0
Investment in operating leases, net	0			0
Intercompany receivables from bank subsidiary	0			0
Intercompany receivables from nonbank subsidiary	0			0
Investment in bank subsidiaries	16,057			16,962
Investment in nonbank subsidiaries	0			0
Premiums receivable and other insurance assets	0			0
Other assets	0			0
Assets	16,057			16,962
Deposit liabilities, noninterest-bearing	0			0
Deposit liabilities, interest-bearing	0			0
Deposit liabilities	0			0
Short-term borrowings	0			0
Long-term debt	0			0
Intercompany debt payable to bank subsidiaries	0			0
Intercompany debt payable to nonbank subsidiaries	0			0
Intercompany payables to bank subsidiary	0			0
Intercompany payables to nonbank subsidiaries	0			0
Interest payable	0			0
Unearned insurance premiums and service revenue	0			0
Accrued expenses and other liabilities	0			0
Total liabilities	0			0
Equity	16,057			16,962
Total liabilities and equity	16,057			16,962
Nonguarantors
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents, noninterest-bearing	752			770
Cash and cash equivalents, interest-bearing	2,965			3,403
Cash and cash equivalents	4,286			4,868	4,903
Equity securities	514			518
Available-for-sale securities	24,122			22,303
Held-to-maturity securities	2,269			1,973
Loans held-for-sale, net	425			108
Finance receivables and loans, net	122,226			115,459
Intercompany loans to nonbank subsidiary	405			408
Allowance for loan losses	1,150			1,091
Total finance receivables and loans, net	121,481			114,776
Investment in operating leases, net	8,571			8,722
Intercompany receivables from bank subsidiary	0			0
Intercompany receivables from nonbank subsidiary	121			77
Investment in bank subsidiaries	0			0
Investment in nonbank subsidiaries	0			0
Premiums receivable and other insurance assets	2,291			2,082
Other assets	4,999			5,105
Assets	169,079			160,532
Deposit liabilities, noninterest-bearing	180			108
Deposit liabilities, interest-bearing	101,196			93,136
Deposit liabilities	102,289			94,383
Short-term borrowings	4,763			8,242
Long-term debt	31,431			26,260
Intercompany debt payable to bank subsidiaries	0			0
Intercompany debt payable to nonbank subsidiaries	821			879
Intercompany payables to bank subsidiary	0			0
Intercompany payables to nonbank subsidiaries	81			127
Interest payable	470			175
Unearned insurance premiums and service revenue	3,020			2,604
Accrued expenses and other liabilities	3,148			2,790
Total liabilities	146,023			135,460
Equity	23,056			25,072
Total liabilities and equity	169,079			160,532
Affiliated entity
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents, interest-bearing	0			0
Deposit liabilities, interest-bearing	0			0
Affiliated entity | Parent
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents, interest-bearing	913			1,138
Deposit liabilities, interest-bearing	0			0
Affiliated entity | Guarantors
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents, interest-bearing	0			0
Deposit liabilities, interest-bearing	0			0
Affiliated entity | Nonguarantors
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents, interest-bearing	569			695
Deposit liabilities, interest-bearing	913			1,139
Consolidating adjustments
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents, noninterest-bearing	0			0
Cash and cash equivalents, interest-bearing	0			0
Cash and cash equivalents	(1,482)			(1,833)	$ (2,053)
Equity securities	0			0
Available-for-sale securities	0			0
Held-to-maturity securities	(23)			(74)
Loans held-for-sale, net	0			0
Finance receivables and loans, net	0			0
Intercompany loans to nonbank subsidiary	(1,226)			(1,287)
Allowance for loan losses	0			0
Total finance receivables and loans, net	(1,226)			(1,287)
Investment in operating leases, net	0			0
Intercompany receivables from bank subsidiary	(113)			(80)
Intercompany receivables from nonbank subsidiary	(165)			(148)
Investment in bank subsidiaries	(32,114)			(33,924)
Investment in nonbank subsidiaries	(6,999)			(8,111)
Premiums receivable and other insurance assets	0			(35)
Other assets	(1,423)			(1,649)
Assets	(43,545)			(47,141)
Deposit liabilities, noninterest-bearing	0			0
Deposit liabilities, interest-bearing	0			0
Deposit liabilities	(913)			(1,139)
Short-term borrowings	0			0
Long-term debt	0			0
Intercompany debt payable to bank subsidiaries	(23)			(74)
Intercompany debt payable to nonbank subsidiaries	(1,795)			(1,982)
Intercompany payables to bank subsidiary	(45)			(4)
Intercompany payables to nonbank subsidiaries	(198)			(259)
Interest payable	0			0
Unearned insurance premiums and service revenue	0			0
Accrued expenses and other liabilities	(1,458)			(1,649)
Total liabilities	(4,432)			(5,107)
Equity	(39,113)			(42,034)
Total liabilities and equity	(43,545)			(47,141)
Consolidating adjustments | Affiliated entity
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents, interest-bearing	(1,482)			(1,833)
Deposit liabilities, interest-bearing	$ (913)			$ (1,139)