Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Apr. 01, 2019	Jan. 01, 2019	Apr. 01, 2018	Jan. 01, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 183	$ 172	$ 362	$ 344
Other revenue	395	364	861	718
Insurance commissions	117	109	231	219
Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	212	192	499	374
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	21	18	43	37
Other revenue	34	19	59	52
Corporate and Other | Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	13	1	16	15
Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	134	125	265	248
Unearned revenue, remaining performance obligation, amount	2,800	2,600	2,800	2,600	$ 2,700	$ 2,600	$ 2,500	$ 2,500
Unearned revenue, revenue recognized	202	195	401	389
Deferred commissions	1,600	1,500	1,600	1,500	$ 1,600	$ 1,500	$ 1,400	$ 1,400
Insurance commissions	114	106	225	209
Noninsurance contracts | Corporate and Other
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	19	21	37	44
Remarketing fee income | Corporate and Other
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	17	15	34	31
Brokerage commissions and other revenue | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	17	15	34	31
Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	3	9	6
Deposit account and other banking fees | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	3	9	6
Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	4	7	8
Brokered/agent commissions | Corporate and Other
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	5	4	10	7
Other | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	24	25	47	50
Other revenue	61	63	129	129
Automotive Finance operations | Operating Segments | Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	37	38	82	79
Automotive Finance operations | Noninsurance contracts | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Automotive Finance operations | Remarketing fee income | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	19	21	37	44
Automotive Finance operations | Brokerage commissions and other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	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	0	0
Automotive Finance operations | Brokered/agent commissions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Automotive Finance operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5	4	10	6
Automotive Finance operations | Sale of off-lease vehicles
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	23	16	38	34
Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	138	129	272	257
Other revenue	286	266	646	512
Insurance operations | Operating Segments | Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	148	137	374	255
Insurance operations | Noninsurance contracts | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	134	125	265	248
Insurance operations | Remarketing fee income | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Insurance operations | Brokerage commissions and other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Insurance operations | Deposit account and other banking fees | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Insurance operations | Brokered/agent commissions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4	4	7	8
Insurance operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	1
Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other revenue	4	2	6	3
Mortgage Finance operations | Operating Segments | Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	4	2	6	3
Mortgage Finance operations | Noninsurance contracts | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Mortgage Finance operations | Remarketing fee income | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Mortgage Finance operations | Brokerage commissions and other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	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	0	0
Mortgage Finance operations | Brokered/agent commissions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Mortgage Finance operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other revenue	10	14	21	22
Corporate Finance operations | Operating Segments | Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	10	14	21	22
Corporate Finance operations | Noninsurance contracts | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Corporate Finance operations | Remarketing fee income | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Corporate Finance operations | Brokerage commissions and other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	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	0	0
Corporate Finance operations | Brokered/agent commissions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Corporate Finance operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	$ 0	0	$ 0
2019 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	380		380
2020 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	705		705
2021 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	605		605
2022 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	473		473
2023 and thereafter | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	$ 595		$ 595