Parent and Guarantor Condensed Consolidating Financial Statements (Statement of Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	$ 1,097	$ 1,175
Purchases of Equity Securities	(374)	(137)
Proceeds from Sales of Equity Securities	220	314
Payments to Acquire Available-for-sale Securities	(2,360)	(2,696)
Proceeds from Sale of Available-for-sale Securities	328	731
Proceeds from maturities and repayment of available-for-sale securities	795	589
Payments to Acquire Held-to-maturity Securities	(155)	(215)
Proceeds from Sale and Maturity of Held-to-maturity Securities	35	5
Payments to Acquire Finance Receivables	(1,497)	(405)
Proceeds from Sale of Loans Held-for-investment	0	1,164
Originations and repayments of loans held-for-investment and other	(1,300)	(1,174)
Gain (Loss) on Sales of Loans, Net	1	14
Purchases Of Operating Lease Assets	(969)	(893)
Disposals of Operating Lease Assets	976	1,545
Proceeds from Contributions from Parent	0	0
Returns Of Contributed Capital	0	0
Increase (Decrease) in Non marketable equity securities	(19)	213
Payments for (Proceeds from) Other Investing Activities	(82)	(56)
Net Cash Provided by (Used in) Investing Activities	(4,402)	(1,015)
Proceeds from (Repayments of) Short-term Debt	(1,848)	(4,303)
Increase (Decrease) in Deposit Assets	4,173	5,451
Proceeds from Issuance of Long-term Debt	6,665	4,488
Repayments of Long-term Debt	(5,771)	(7,573)
Payments for Repurchase of Common Stock	(185)	(169)
Payments of Dividends	(58)	(38)
Capital contributions from parent	0	0
Net Cash Provided by (Used in) Financing Activities	2,976	(2,144)
Effect of Exchange Rate on Cash and Cash Equivalents	(2)	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(331)	(1,984)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	4,938	5,897	$ 5,269	$ 7,881
Cash and Cash Equivalents, at Carrying Value	3,721	4,302	4,252
Restricted Cash	1,217	1,595
Parent Company [Member]
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	456	(149)
Purchases of Equity Securities	0	0
Proceeds from Sales of Equity Securities	0	0
Payments to Acquire Available-for-sale Securities	0	0
Proceeds from Sale of Available-for-sale Securities	0	0
Proceeds from maturities and repayment of available-for-sale securities	0	0
Payments to Acquire Held-to-maturity Securities	0	0
Proceeds from Sale and Maturity of Held-to-maturity Securities	0	0
Payments to Acquire Finance Receivables	0	(15)
Proceeds from Sale of Loans Held-for-investment	820	0
Originations and repayments of loans held-for-investment and other	432	931
Gain (Loss) on Sales of Loans, Net	28	(2)
Purchases Of Operating Lease Assets	0	0
Disposals of Operating Lease Assets	4	1
Proceeds from Contributions from Parent	(49)	(83)
Returns Of Contributed Capital	38	645
Increase (Decrease) in Non marketable equity securities	0	0
Payments for (Proceeds from) Other Investing Activities	(3)	(26)
Net Cash Provided by (Used in) Investing Activities	819	2,600
Proceeds from (Repayments of) Short-term Debt	(214)	1,278
Increase (Decrease) in Deposit Assets	(6)	(82)
Proceeds from Issuance of Long-term Debt	15	330
Repayments of Long-term Debt	(1,152)	(2,870)
Payments for Repurchase of Common Stock	(185)	(169)
Payments of Dividends	(58)	(38)
Capital contributions from parent	0	0
Net Cash Provided by (Used in) Financing Activities	(1,727)	(1,754)
Effect of Exchange Rate on Cash and Cash Equivalents	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(452)	697
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	943	1,686	1,395	989
Cash and Cash Equivalents, at Carrying Value	796	1,490	1,217
Restricted Cash	147	196
Guarantor Subsidiaries [Member]
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	1,000	0
Purchases of Equity Securities	0	0
Proceeds from Sales of Equity Securities	0	0
Payments to Acquire Available-for-sale Securities	0	0
Proceeds from Sale of Available-for-sale Securities	0	0
Proceeds from maturities and repayment of available-for-sale securities	0	0
Payments to Acquire Held-to-maturity Securities	0	0
Proceeds from Sale and Maturity of Held-to-maturity Securities	0	0
Payments to Acquire Finance Receivables	0	0
Proceeds from Sale of Loans Held-for-investment	0	0
Originations and repayments of loans held-for-investment and other	0	0
Gain (Loss) on Sales of Loans, Net	0	0
Purchases Of Operating Lease Assets	0	0
Disposals of Operating Lease Assets	0	0
Proceeds from Contributions from Parent	(6)	0
Returns Of Contributed Capital	0	0
Increase (Decrease) in Non marketable equity securities	0	0
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Provided by (Used in) Investing Activities	(6)	0
Proceeds from (Repayments of) Short-term Debt	0	0
Increase (Decrease) in Deposit Assets	0	0
Proceeds from Issuance of Long-term Debt	0	0
Repayments of Long-term Debt	0	0
Payments for Repurchase of Common Stock	0	0
Payments of Dividends	0	0
Capital contributions from parent	6	0
Net Cash Provided by (Used in) Financing Activities	(994)	0
Effect of Exchange Rate on Cash and Cash Equivalents	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0	0	0
Cash and Cash Equivalents, at Carrying Value	0	0	0
Restricted Cash	0	0
Non-Guarantor Subsidiaries [Member]
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	1,812	1,284
Purchases of Equity Securities	(374)	(137)
Proceeds from Sales of Equity Securities	220	314
Payments to Acquire Available-for-sale Securities	(2,360)	(2,696)
Proceeds from Sale of Available-for-sale Securities	328	731
Proceeds from maturities and repayment of available-for-sale securities	795	589
Payments to Acquire Held-to-maturity Securities	(155)	(215)
Proceeds from Sale and Maturity of Held-to-maturity Securities	35	5
Payments to Acquire Finance Receivables	(2,317)	(390)
Proceeds from Sale of Loans Held-for-investment	0	1,164
Originations and repayments of loans held-for-investment and other	(1,732)	(1,145)
Gain (Loss) on Sales of Loans, Net	1	16
Purchases Of Operating Lease Assets	(969)	(893)
Disposals of Operating Lease Assets	972	1,544
Proceeds from Contributions from Parent	0	0
Returns Of Contributed Capital	0	0
Increase (Decrease) in Non marketable equity securities	(19)	213
Payments for (Proceeds from) Other Investing Activities	(80)	64
Net Cash Provided by (Used in) Investing Activities	(5,646)	(421)
Proceeds from (Repayments of) Short-term Debt	(1,634)	(5,581)
Increase (Decrease) in Deposit Assets	3,776	5,533
Proceeds from Issuance of Long-term Debt	6,650	3,196
Repayments of Long-term Debt	(4,619)	(4,703)
Payments for Repurchase of Common Stock	0	0
Payments of Dividends	0	0
Capital contributions from parent	49	83
Net Cash Provided by (Used in) Financing Activities	3,436	(3,304)
Effect of Exchange Rate on Cash and Cash Equivalents	(2)	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(400)	(2,441)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	5,307	4,852	5,707	7,293
Cash and Cash Equivalents, at Carrying Value	4,237	3,453	4,868
Restricted Cash	1,070	1,399
Consolidation, Eliminations [Member]
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	(2,171)	40
Purchases of Equity Securities	0	0
Proceeds from Sales of Equity Securities	0	0
Payments to Acquire Available-for-sale Securities	0	0
Proceeds from Sale of Available-for-sale Securities	0	0
Proceeds from maturities and repayment of available-for-sale securities	0	0
Payments to Acquire Held-to-maturity Securities	0	0
Proceeds from Sale and Maturity of Held-to-maturity Securities	0	0
Payments to Acquire Finance Receivables	820	0
Proceeds from Sale of Loans Held-for-investment	(820)	0
Originations and repayments of loans held-for-investment and other	0	(960)
Gain (Loss) on Sales of Loans, Net	(28)	0
Purchases Of Operating Lease Assets	0	0
Disposals of Operating Lease Assets	0	0
Proceeds from Contributions from Parent	55	83
Returns Of Contributed Capital	(38)	(645)
Increase (Decrease) in Non marketable equity securities	0	0
Payments for (Proceeds from) Other Investing Activities	1	(94)
Net Cash Provided by (Used in) Investing Activities	431	(3,194)
Proceeds from (Repayments of) Short-term Debt	0	0
Increase (Decrease) in Deposit Assets	403	0
Proceeds from Issuance of Long-term Debt	0	962
Repayments of Long-term Debt	0	0
Payments for Repurchase of Common Stock	0	0
Payments of Dividends	0	0
Capital contributions from parent	(55)	(83)
Net Cash Provided by (Used in) Financing Activities	2,261	2,914
Effect of Exchange Rate on Cash and Cash Equivalents	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	521	(240)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	(1,312)	(641)	(1,833)	$ (401)
Cash and Cash Equivalents, at Carrying Value	(1,312)	(641)	$ (1,833)
Restricted Cash	0	0
Affiliated Entity [Member]
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Payments for (Proceeds from) Investments	0	0
Gain (Loss) on Sales of Loans, Net	0	0
Net Change in Debt, Intercompany	0	0
Dividends paid and returns of contributed capital, Intercompany	0	0
Affiliated Entity [Member] | Parent Company [Member]
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Payments for (Proceeds from) Investments	0	1
Gain (Loss) on Sales of Loans, Net	(423)	1,146
Net Change in Debt, Intercompany	(127)	(203)
Dividends paid and returns of contributed capital, Intercompany	0	0
Affiliated Entity [Member] | Guarantor Subsidiaries [Member]
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Payments for (Proceeds from) Investments	0	0
Gain (Loss) on Sales of Loans, Net	0	0
Net Change in Debt, Intercompany	0	0
Dividends paid and returns of contributed capital, Intercompany	(1,000)	0
Affiliated Entity [Member] | Non-Guarantor Subsidiaries [Member]
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Payments for (Proceeds from) Investments	9	261
Gain (Loss) on Sales of Loans, Net	1	170
Net Change in Debt, Intercompany	422	(1,146)
Dividends paid and returns of contributed capital, Intercompany	(1,208)	(686)
Affiliated Entity [Member] | Consolidation, Eliminations [Member]
Parent and Guarantor Consolidating Statement of Cash Flows [Line Items]
Payments for (Proceeds from) Investments	(9)	(262)
Gain (Loss) on Sales of Loans, Net	422	(1,316)
Net Change in Debt, Intercompany	(295)	1,349
Dividends paid and returns of contributed capital, Intercompany	$ 2,208	$ 686