Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 218	$ 186
Other revenue	565	266
Insurance premiums and service revenue earned	280	277
Other gain (loss) on investments, net	123	(79)
Remarketing gains, net	64	2
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	26	17
Other revenue	58	59
Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	33	22
Other revenue	62	47
Remarketing gains, net	64	2
Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	159	147
Other revenue	379	137
Insurance premiums and service revenue earned		132
Other gain (loss) on investments, net		142
Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Other revenue	40	10
Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Other revenue	26	13
Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	155	143
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	155	143
Unearned revenue, remaining performance obligation, amount	3,000	2,900	$ 3,000	$ 2,900
Unearned revenue, revenue recognized	225	214
Capitalized contract cost, net	1,800	1,700	$ 1,800	$ 1,700
Capitalized contract cost, amortization	132	125
Noninsurance contracts | Insurance operations | Operating Segments | 2021
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	620
Noninsurance contracts | Insurance operations | Operating Segments | 2022
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	757
Noninsurance contracts | Insurance operations | Operating Segments | 2023
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	659
Noninsurance contracts | Insurance operations | Operating Segments | 2024
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	483
Noninsurance contracts | Insurance operations | Operating Segments | 2025 and thereafter
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	513
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	27	17
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	27	17
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	20	13
Brokerage commissions and other revenue | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	20	13
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
Deposits and Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6	4
Deposits and Banking | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6	4
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
Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	4
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	4	4
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
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6	5
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	6	5
Other | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
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	347	80
Out of scope of Topic 606 | Corporate and Other
Disaggregation of Revenue [Line Items]
Other revenue	32	42
Out of scope of Topic 606 | Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	29	25
Out of scope of Topic 606 | Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	220	(10)
Out of scope of Topic 606 | Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	40	10
Out of scope of Topic 606 | Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	$ 26	$ 13