Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jul. 01, 2019	Jan. 01, 2019	Jul. 01, 2018	Jan. 01, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 184	$ 173	$ 546	$ 517
Other revenue	413	398	1,274	1,116
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	19	18	62	55
Other revenue	46	20	105	72
Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	138	129	403	377
Unearned revenue, remaining performance obligation, amount	2,800	2,600	2,800	2,600	$ 2,800	$ 2,600	$ 2,600	$ 2,500
Unearned revenue, revenue recognized	206	199	607	588
Capitalized Contract Cost, Net	1,700	1,500	1,700	1,500	$ 1,600	$ 1,500	$ 1,500	$ 1,400
Capitalized Contract Cost, Amortization	119	108	344	317
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	19	56	63
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	16	15	50	46
Brokerage commissions and other revenue | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	16	15	50	46
Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3	3	12	9
Deposit account and other banking fees | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3	3	12	9
Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3	3	10	11
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	15	11
Other | Corporate and Other
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	229	225	728	599
Out of scope of Topic 606 | Corporate and Other
Disaggregation of Revenue [Line Items]
Other revenue	27	2	43	17
Automotive Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	24	23	71	73
Other revenue	59	80	188	209
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	19	56	63
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	15	10
Automotive Finance operations | Out of scope of Topic 606 | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	35	57	117	136
Automotive Finance operations | Sale of off-lease vehicles
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	28	27	66	61
Insurance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	141	132	413	389
Other revenue	289	282	935	794
Insurance operations | Noninsurance contracts | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	138	129	403	377
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	3	3	10	11
Insurance operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	1
Insurance operations | Out of scope of Topic 606 | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	148	150	522	405
Mortgage Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other revenue	10	2	16	5
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
Mortgage Finance operations | Out of scope of Topic 606 | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	10	2	16	5
Corporate Finance operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other revenue	9	14	30	36
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
Corporate Finance operations | Out of scope of Topic 606 | Operating Segments
Disaggregation of Revenue [Line Items]
Other revenue	9	$ 14	30	$ 36
2019 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	197		197
2020 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	737		737
2021 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	646		646
2022 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	523		523
2023 and thereafter | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	$ 720		$ 720