Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 186	$ 179
Other revenue	266	466
Remarketing (gains) and losses, net	2	15
Insurance premiums and service revenue earned	277	261
Other gain on investments, net	(79)	108
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	17	22
Other revenue	59	25
Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	143	131
Unearned revenue, remaining performance obligation, amount	2,900	2,700	$ 2,900	$ 2,600
Unearned revenue, revenue recognized	214	199
Capitalized contract cost, net	1,700	1,600	$ 1,700	$ 1,500
Capitalized contract cost, amortization	125	111
Noninsurance contracts | Corporate and Other
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	17	18
Remarketing fee income | Corporate and Other
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	13	17
Brokerage commissions and other revenue | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	13	17
Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	5
Deposit account and other banking fees | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	5
Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	3
Brokered/agent commissions | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5	5
Other | Corporate and Other
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	80	287
Out of scope of Topic 606 | Corporate and Other
Disaggregation of Revenue [Line Items]
Other revenue	42	3
Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	22	23
Other revenue	47	68
Automotive Finance operations | Noninsurance contracts | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Automotive Finance operations | Remarketing fee income | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	17	18
Automotive Finance operations | Brokerage commissions and other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Automotive Finance operations | Deposit account and other banking fees | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Automotive Finance operations | Brokered/agent commissions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Automotive Finance operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5	5
Automotive Finance operations | Out of scope of Topic 606 | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	25	45
Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	147	134
Other revenue	137	360
Insurance premiums and service revenue earned	132
Other gain on investments, net	142
Insurance operations | Noninsurance contracts | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	143	131
Insurance operations | Remarketing fee income | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Insurance operations | Brokerage commissions and other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Insurance operations | Deposit account and other banking fees | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Insurance operations | Brokered/agent commissions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	3
Insurance operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Insurance operations | Out of scope of Topic 606 | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	(10)	226
Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Other revenue	10	2
Mortgage Finance operations | Noninsurance contracts | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mortgage Finance operations | Remarketing fee income | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mortgage Finance operations | Brokerage commissions and other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mortgage Finance operations | Deposit account and other banking fees | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mortgage Finance operations | Brokered/agent commissions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mortgage Finance operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mortgage Finance operations | Out of scope of Topic 606 | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	10	2
Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Other revenue	13	11
Corporate Finance operations | Noninsurance contracts | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations | Remarketing fee income | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations | Brokerage commissions and other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations | Deposit account and other banking fees | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations | Brokered/agent commissions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations | Out of scope of Topic 606 | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	13	$ 11
2020 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	591
2021 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	714
2022 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	609
2023 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	476
2024 and thereafter | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	$ 498