Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Jan. 01, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 179	$ 172
Other revenue	466	354
Insurance commissions	114	110
Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	287	182
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	22	19
Other revenue	25	33
Corporate and Other | Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	3	14
Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	131	123
Unearned revenue, remaining performance obligation, amount	2,700	2,500	$ 2,600	$ 2,500
Unearned revenue, revenue recognized	199	194
Deferred commissions	1,600	1,400	$ 1,500
Insurance commissions	111	103
Noninsurance contracts | 2019
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	554
Noninsurance contracts | 2020
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	672
Noninsurance contracts | 2021
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	562
Noninsurance contracts | 2022
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	424
Noninsurance contracts | 2023 and thereafter
Disaggregation of Revenue [Line Items]
Unearned revenue, remaining performance obligation, amount	477
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	18	23
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	17	16
Brokerage commissions and other revenue | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	17	16
Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5	3
Deposit account and other banking fees | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5	3
Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3	4
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	3
Other | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Automotive Finance operations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	23	25
Other revenue	68	66
Automotive Finance operations | Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	45	41
Automotive Finance operations | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Automotive Finance operations | Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	18	23
Automotive Finance operations | Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Automotive Finance operations | Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Automotive Finance operations | Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Automotive Finance operations | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5	2
Automotive Finance operations | Sale of off-lease vehicles
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	15	18
Insurance operations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	134	128
Other revenue	360	246
Insurance operations | Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	226	118
Insurance operations | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	131	123
Insurance operations | Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Insurance operations | Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Insurance operations | Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Insurance operations | Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3	4
Insurance operations | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	1
Mortgage Finance operations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Other revenue	2	1
Mortgage Finance operations | Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	2	1
Mortgage Finance operations | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mortgage Finance operations | Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mortgage Finance operations | Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mortgage Finance operations | Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mortgage Finance operations | Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mortgage Finance operations | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Other revenue	11	8
Corporate Finance operations | Out of scope of Topic 606
Disaggregation of Revenue [Line Items]
Other revenue	11	8
Corporate Finance operations | Noninsurance contracts
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations | Remarketing fee income
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations | Brokerage commissions and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations | Deposit account and other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations | Brokered/agent commissions
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Corporate Finance operations | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 0	$ 0