Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									$ 753	$ 727
Other revenue	$ 678	$ 484	$ 555	$ 266	$ 487	$ 413	$ 395	$ 466	1,983	1,761	$ 1,414
Remarketing gains, net									127	69	90
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									64	77
Other revenue									298	171	119
Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									88	93
Other revenue									204	249	269
Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									601	557
Other revenue									1,334	1,274	981
Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Other revenue									102	22	7
Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Other revenue									45	45	38
Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									584	542
Unearned revenue, remaining performance obligation, amount	3,000				2,900				3,000	2,900	2,600
Unearned revenue, revenue recognized									866	816
Capitalized contract cost, net	1,800				$ 1,700				1,800	1,700	$ 1,500
Capitalized contract cost, amortization									498	463
Noninsurance contracts | 2021
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	808								808
Noninsurance contracts | 2022
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	722								722
Noninsurance contracts | 2023
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	613								613
Noninsurance contracts | 2024
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	433								433
Noninsurance contracts | 2025 and thereafter
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	$ 416								416
Noninsurance contracts | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Noninsurance contracts | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Noninsurance contracts | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									584	542
Noninsurance contracts | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Noninsurance contracts | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									73	74
Remarketing fee income | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Remarketing fee income | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									73	74
Remarketing fee income | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Remarketing fee income | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Remarketing fee income | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									52	61
Brokerage commissions and other revenue | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									52	61
Brokerage commissions and other revenue | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Brokerage commissions and other revenue | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Brokerage commissions and other revenue | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Brokerage commissions and other revenue | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									16	14
Brokered/agent commissions | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Brokered/agent commissions | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Brokered/agent commissions | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									16	14
Brokered/agent commissions | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Brokered/agent commissions | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Deposits and Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									12	16
Deposits and Banking | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									12	16
Deposits and Banking | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Deposits and Banking | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Deposits and Banking | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Deposits and Banking | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									16	20
Other | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Other | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									15	19
Other | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									1	1
Other | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Other | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0	0
Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue									1,230	1,034
Out of scope of Topic 606 | Corporate and Other
Disaggregation of Revenue [Line Items]
Other revenue									234	94
Out of scope of Topic 606 | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue									116	156
Out of scope of Topic 606 | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue									733	717
Out of scope of Topic 606 | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue									102	22
Out of scope of Topic 606 | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue									$ 45	$ 45