Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Insurance commissions									$ 440	$ 415	$ 389
Revenue from contracts with customers									689
Other revenue	$ 298	$ 398	$ 364	$ 354	$ 379	$ 381	$ 388	$ 396	1,414	1,544	1,530
Noninsurance contracts
Disaggregation of Revenue [Line Items]
Contract with customer, unearned revenue	289								289
Unearned revenue, revenue recognized									91
Unearned revenue, remaining performance obligation, amount	2,600								2,600
Deferred commissions	1,500								1,500
Insurance commissions									427
Revenue from contracts with customers									506
Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									79
Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									62
Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									15
Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Gross interchange fee income									12
Interchange expense									10
Revenue from contracts with customers									13
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									14
Automotive Finance operations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									92
Other revenue									269	355	306
Automotive Finance operations | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Automotive Finance operations | Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									79
Automotive Finance operations | Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Automotive Finance operations | Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Automotive Finance operations | Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Automotive Finance operations | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									13
Automotive Finance operations | Sale of off-lease vehicles
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									90
Insurance operations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									522
Other revenue									981	1,059	1,036
Insurance operations | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									506
Insurance operations | Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Insurance operations | Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Insurance operations | Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									15
Insurance operations | Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Insurance operations | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									1
Mortgage Finance operations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Other revenue									7	4	0
Mortgage Finance operations | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Mortgage Finance operations | Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Mortgage Finance operations | Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Mortgage Finance operations | Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Mortgage Finance operations | Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Mortgage Finance operations | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Corporate Finance operations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Other revenue									38	45	26
Corporate Finance operations | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Corporate Finance operations | Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Corporate Finance operations | Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Corporate Finance operations | Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Corporate Finance operations | Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Corporate Finance operations | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
2019 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	721								721
2020 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	639								639
2021 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	519								519
2022 | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	379								379
2023 and thereafter | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	$ 386								386
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									75
Other revenue									119	$ 81	$ 162
Corporate and Other | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Corporate and Other | Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Corporate and Other | Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									62
Corporate and Other | Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Corporate and Other | Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									13
Corporate and Other | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									0
Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue									725
Out of scope of Topic 606 | Automotive Finance operations
Disaggregation of Revenue [Line Items]
Other revenue									177
Out of scope of Topic 606 | Insurance operations
Disaggregation of Revenue [Line Items]
Other revenue									459
Out of scope of Topic 606 | Mortgage Finance operations
Disaggregation of Revenue [Line Items]
Other revenue									7
Out of scope of Topic 606 | Corporate Finance operations
Disaggregation of Revenue [Line Items]
Other revenue									38
Out of scope of Topic 606 | Corporate and Other
Disaggregation of Revenue [Line Items]
Other revenue									$ 44