Other Assets (Schedule of Other Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Mar. 31, 2024	Dec. 31, 2022
Finite-Lived Intangible Assets [Line Items]
Property and equipment at cost	$ 2,153	$ 2,160
Accumulated depreciation	(871)	(890)
Net property and equipment	1,282	1,270
Proportional amortization investments	1,866	1,877
Net deferred tax assets	1,224	1,323
Accrued interest, fees, and rent receivables	935	941
Nonmarketable equity investments	886	747
Goodwill	669	669	$ 822
Equity-method investments	651	638
Restricted cash held for securitization trusts	407	397
Other accounts receivable	189	193
Operating lease right-of-use assets	90	98
Restricted cash and cash equivalents	87	90
Net intangible assets	73	67
Other assets	1,036	1,038
Total other assets	$ 9,395	$ 9,348
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Total other assets	Total other assets
Capitalized software
Finite-Lived Intangible Assets [Line Items]
Property, plant and equipment, disposals	$ 295
Disposals, accumulated depreciation	$ 295