Parent and Guarantor Condensed Consolidating Financial Statements (Statement of Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net cash provided by (used in) operating activities			$ 3,344	$ 3,373
Purchases of equity securities			(652)	(612)
Proceeds from sales of equity securities			715	728
Purchases of available-for-sale securities			(5,669)	(8,410)
Proceeds from sale of available-for-sale securities			637	2,198
Proceeds from repayments of available-for-sale securities			2,509	2,002
Purchases of held-to-maturity securities			(436)	(709)
Proceeds from repayments of held-to-maturity securities			107	32
Purchases of finance receivables and loans held-for-investment			(4,778)	(3,125)
Proceeds from sales of finance receivables and loans initially held-for-investment			53	1,323
Originations and repayments of finance receivables and loans held-for-investment and other, net			(558)	1,021
Net change in intercompany loans	$ 17	$ 15	19	65
Purchases Of Operating Lease Assets			(2,991)	(2,844)
Disposals of operating lease assets			2,461	4,409
Capital contributions to subsidiaries			0	0
Returns of contributed capital			0	0
Net change in nonmarketable equity investments			(3)	(20)
Payments for (proceeds from) other, net			(241)	(155)
Net cash provided by investing activities			(8,846)	(4,162)
Net change in short-term borrowings			(4,074)	(2,500)
Net (decrease) increase in deposits			8,063	11,050
Proceeds from issuance of long-term debt			14,756	13,302
Repayments of long-term debt			(12,994)	(22,376)
Repurchase of common stock			(630)	(563)
Dividends paid			(179)	(130)
Capital contributions from parent			0	0
Net cash provided by (used in) financing activities			4,942	(1,217)
Effect of exchange-rate changes on cash and cash equivalents and restricted cash			(2)	3
Net increase (decrease) in cash and cash equivalents and restricted cash			(562)	(2,003)
Cash and cash equivalents and restricted cash	4,707	5,878	4,707	5,878	$ 5,269	$ 7,881
Cash and cash equivalents	3,772	4,424	3,772	4,424	4,252
Restricted cash included in other assets on the Condensed Consolidated Balance Sheet	935	1,454	935	1,454
Parent
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net cash provided by (used in) operating activities			1,417	3,701
Purchases of equity securities			0	0
Proceeds from sales of equity securities			0	0
Purchases of available-for-sale securities			0	0
Proceeds from sale of available-for-sale securities			0	0
Proceeds from repayments of available-for-sale securities			0	0
Purchases of held-to-maturity securities			0	0
Proceeds from repayments of held-to-maturity securities			0	0
Purchases of finance receivables and loans held-for-investment			(131)	(35)
Proceeds from sales of finance receivables and loans initially held-for-investment			983	96
Originations and repayments of finance receivables and loans held-for-investment and other, net			2,092	259
Net change in intercompany loans	16	9	44	39
Purchases Of Operating Lease Assets			0	0
Disposals of operating lease assets			9	7
Capital contributions to subsidiaries			(58)	(1,200)
Returns of contributed capital			222	1,031
Net change in nonmarketable equity investments			(14)	0
Payments for (proceeds from) other, net			1	(20)
Net cash provided by investing activities			3,149	2,304
Net change in short-term borrowings			(596)	(245)
Net (decrease) increase in deposits			(9)	(153)
Proceeds from issuance of long-term debt			51	355
Repayments of long-term debt			(3,393)	(4,125)
Repurchase of common stock			(630)	(563)
Dividends paid			(179)	(130)
Capital contributions from parent			0	0
Net cash provided by (used in) financing activities			(4,899)	(5,227)
Effect of exchange-rate changes on cash and cash equivalents and restricted cash			0	0
Net increase (decrease) in cash and cash equivalents and restricted cash			(333)	778
Cash and cash equivalents and restricted cash	1,062	1,767	1,062	1,767	1,395	989
Cash and cash equivalents	968	1,574	968	1,574	1,217
Restricted cash included in other assets on the Condensed Consolidated Balance Sheet	94	193	94	193
Guarantors
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net cash provided by (used in) operating activities			2,050	2,900
Purchases of equity securities			0	0
Proceeds from sales of equity securities			0	0
Purchases of available-for-sale securities			0	0
Proceeds from sale of available-for-sale securities			0	0
Proceeds from repayments of available-for-sale securities			0	0
Purchases of held-to-maturity securities			0	0
Proceeds from repayments of held-to-maturity securities			0	0
Purchases of finance receivables and loans held-for-investment			0	0
Proceeds from sales of finance receivables and loans initially held-for-investment			0	0
Originations and repayments of finance receivables and loans held-for-investment and other, net			0	0
Net change in intercompany loans	0	0	0	0
Purchases Of Operating Lease Assets			0	0
Disposals of operating lease assets			0	0
Capital contributions to subsidiaries			(6)	0
Returns of contributed capital			0	0
Net change in nonmarketable equity investments			0	0
Payments for (proceeds from) other, net			0	0
Net cash provided by investing activities			(6)	0
Net change in short-term borrowings			0	0
Net (decrease) increase in deposits			0	0
Proceeds from issuance of long-term debt			0	0
Repayments of long-term debt			0	0
Repurchase of common stock			0	0
Dividends paid			0	0
Capital contributions from parent			6	0
Net cash provided by (used in) financing activities			(2,044)	(2,900)
Effect of exchange-rate changes on cash and cash equivalents and restricted cash			0	0
Net increase (decrease) in cash and cash equivalents and restricted cash			0	0
Cash and cash equivalents and restricted cash	0	0	0	0	0	0
Cash and cash equivalents	0	0	0	0	0
Restricted cash included in other assets on the Condensed Consolidated Balance Sheet	0	0	0	0
Nonguarantors
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net cash provided by (used in) operating activities			4,366	3,019
Purchases of equity securities			(652)	(612)
Proceeds from sales of equity securities			715	728
Purchases of available-for-sale securities			(5,669)	(8,410)
Proceeds from sale of available-for-sale securities			637	2,198
Proceeds from repayments of available-for-sale securities			2,509	2,002
Purchases of held-to-maturity securities			(436)	(709)
Proceeds from repayments of held-to-maturity securities			107	32
Purchases of finance receivables and loans held-for-investment			(5,577)	(3,090)
Proceeds from sales of finance receivables and loans initially held-for-investment			0	1,227
Originations and repayments of finance receivables and loans held-for-investment and other, net			(2,650)	2,718
Net change in intercompany loans	1	6	3	26
Purchases Of Operating Lease Assets			(2,991)	(2,844)
Disposals of operating lease assets			2,452	4,402
Capital contributions to subsidiaries			0	0
Returns of contributed capital			0	0
Net change in nonmarketable equity investments			11	(20)
Payments for (proceeds from) other, net			(241)	(39)
Net cash provided by investing activities			(11,740)	(1,904)
Net change in short-term borrowings			(3,478)	(2,255)
Net (decrease) increase in deposits			7,846	12,698
Proceeds from issuance of long-term debt			14,705	10,986
Repayments of long-term debt			(9,601)	(18,251)
Repurchase of common stock			0	0
Dividends paid			0	0
Capital contributions from parent			58	1,200
Net cash provided by (used in) financing activities			6,796	(2,247)
Effect of exchange-rate changes on cash and cash equivalents and restricted cash			(2)	3
Net increase (decrease) in cash and cash equivalents and restricted cash			(580)	(1,129)
Cash and cash equivalents and restricted cash	5,127	6,164	5,127	6,164	5,707	7,293
Cash and cash equivalents	4,286	4,903	4,286	4,903	4,868
Restricted cash included in other assets on the Condensed Consolidated Balance Sheet	841	1,261	841	1,261
Consolidating adjustments
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net cash provided by (used in) operating activities			(4,489)	(6,247)
Purchases of equity securities			0	0
Proceeds from sales of equity securities			0	0
Purchases of available-for-sale securities			0	0
Proceeds from sale of available-for-sale securities			0	0
Proceeds from repayments of available-for-sale securities			0	0
Purchases of held-to-maturity securities			0	0
Proceeds from repayments of held-to-maturity securities			0	0
Purchases of finance receivables and loans held-for-investment			930	0
Proceeds from sales of finance receivables and loans initially held-for-investment			(930)	0
Originations and repayments of finance receivables and loans held-for-investment and other, net			0	(1,956)
Net change in intercompany loans	0	0	(28)	0
Purchases Of Operating Lease Assets			0	0
Disposals of operating lease assets			0	0
Capital contributions to subsidiaries			64	1,200
Returns of contributed capital			(222)	(1,031)
Net change in nonmarketable equity investments			0	0
Payments for (proceeds from) other, net			(1)	(96)
Net cash provided by investing activities			(249)	(4,562)
Net change in short-term borrowings			0	0
Net (decrease) increase in deposits			226	(1,495)
Proceeds from issuance of long-term debt			0	1,961
Repayments of long-term debt			0	0
Repurchase of common stock			0	0
Dividends paid			0	0
Capital contributions from parent			(64)	(1,200)
Net cash provided by (used in) financing activities			5,089	9,157
Effect of exchange-rate changes on cash and cash equivalents and restricted cash			0	0
Net increase (decrease) in cash and cash equivalents and restricted cash			351	(1,652)
Cash and cash equivalents and restricted cash	(1,482)	(2,053)	(1,482)	(2,053)	(1,833)	$ (401)
Cash and cash equivalents	(1,482)	(2,053)	(1,482)	(2,053)	$ (1,833)
Restricted cash included in other assets on the Condensed Consolidated Balance Sheet	$ 0	$ 0	0	0
Affiliated entity
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net change in intercompany investment securities			0	0
Net change in intercompany loans			0	0
Net change in intercompany debt			0	0
Dividends paid and returns of contributed capital, intercompany			0	0
Affiliated entity | Parent
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net change in intercompany investment securities			0	7
Net change in intercompany loans			45	2,159
Net change in intercompany debt			(143)	(366)
Dividends paid and returns of contributed capital, intercompany			0	0
Affiliated entity | Guarantors
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net change in intercompany investment securities			0	0
Net change in intercompany loans			0	0
Net change in intercompany debt			0	0
Dividends paid and returns of contributed capital, intercompany			(2,050)	(2,900)
Affiliated entity | Nonguarantors
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net change in intercompany investment securities			51	281
Net change in intercompany loans			(6)	232
Net change in intercompany debt			(73)	(2,166)
Dividends paid and returns of contributed capital, intercompany			(2,661)	(4,459)
Affiliated entity | Consolidating adjustments
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net change in intercompany investment securities			(51)	(288)
Net change in intercompany loans			(39)	(2,391)
Net change in intercompany debt			216	2,532
Dividends paid and returns of contributed capital, intercompany			$ 4,711	$ 7,359