Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 191	$ 184	$ 556	$ 546
Other revenue	484	413	1,305	1,274
Remarketing gains, net	71	28	62	66
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	16	19	48	62
Other revenue	40	46	151	105
Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	23	24	63	71
Other revenue	61	59	148	188
Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	152	141	445	413
Other revenue	338	289	913	935
Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other revenue	36	10	65	16
Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other revenue	9	9	28	30
Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	148	138	433	403
Unearned revenue, remaining performance obligation, amount	3,000	2,800	3,000	2,800	$ 2,900	$ 2,900	$ 2,800	$ 2,600
Unearned revenue, revenue recognized	218	206	643	607
Capitalized contract cost, net	1,800	1,700	1,800	1,700	$ 1,800	$ 1,700	$ 1,600	$ 1,500
Capitalized contract cost, amortization	125	119	371	344
Noninsurance contracts | 2020
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	210		210
Noninsurance contracts | 2021
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	779		779
Noninsurance contracts | 2022
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	689		689
Noninsurance contracts | 2023
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	568		568
Noninsurance contracts | 2024 and thereafter
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	722		722
Noninsurance contracts | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Noninsurance contracts | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Noninsurance contracts | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	148	138	433	403
Noninsurance contracts | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Noninsurance contracts | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	20	19	52	56
Remarketing fee income | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Remarketing fee income | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	20	19	52	56
Remarketing fee income | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Remarketing fee income | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Remarketing fee income | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	12	16	39	50
Brokerage commissions and other revenue | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	12	16	39	50
Brokerage commissions and other revenue | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Brokerage commissions and other revenue | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Brokerage commissions and other revenue | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Brokerage commissions and other revenue | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	3	12	10
Brokered/agent commissions | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Brokered/agent commissions | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Brokered/agent commissions | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	3	12	10
Brokered/agent commissions | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Brokered/agent commissions | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Deposits and Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	3	9	12
Deposits and Banking | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	3	9	12
Deposits and Banking | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Deposits and Banking | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Deposits and Banking | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Deposits and Banking | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3	5	11	15
Other | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3	5	11	15
Other | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	293	229	749	728
Out of scope of Topic 606 | Corporate and Other
Disaggregation of Revenue [Line Items]
Other revenue	24	27	103	43
Out of scope of Topic 606 | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	38	35	85	117
Out of scope of Topic 606 | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	186	148	468	522
Out of scope of Topic 606 | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	36	10	65	16
Out of scope of Topic 606 | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	$ 9	$ 9	$ 28	$ 30