Related Party Transactions - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2025	Mar. 31, 2024
Related Party Transaction [Line Items]
Revenues		$ 4,164,000,000	$ 3,811,000,000
Related party tax payment		$ 0	0
Line of Credit | General Motors | Five Year Revolving Credit Facility
Related Party Transaction [Line Items]
Debt instrument term	5 years	5 years
Current borrowing capacity	$ 10,000,000,000	$ 10,000,000,000
Junior Subordinated Revolving Credit Facility | General Motors
Related Party Transaction [Line Items]
Line of credit facilities - GM related party facility	1,000,000,000.0	1,000,000,000.0
Revolving Credit Facility
Related Party Transaction [Line Items]
Maximum borrowing capacity	2,500,000,000	2,500,000,000
Revolving Credit Facility | Line of Credit | General Motors
Related Party Transaction [Line Items]
Maximum borrowing capacity	$ 14,100,000,000	$ 14,100,000,000
Revolving Credit Facility | Line of Credit | General Motors | Three Year Revolving Credit Facility
Related Party Transaction [Line Items]
Debt instrument term	3 years	3 years
Current borrowing capacity	$ 4,100,000,000	$ 4,100,000,000
Revolving Credit Facility | Line of Credit | General Motors | 364 Day Revolving Credit Facilities
Related Party Transaction [Line Items]
Maximum borrowing capacity	$ 2,000,000,000.0	$ 2,000,000,000.0
Debt instrument term		364 days
GM
Related Party Transaction [Line Items]
Revenues		$ 704,000,000	$ 777,000,000