Related Party Transactions - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2025	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Related Party Transaction [Line Items]
Revenues		$ 4,337,000,000	$ 4,031,000,000	$ 12,755,000,000	$ 11,760,000,000
Related party tax payment				$ 0	0
General Motors | Line of Credit | 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
General Motors | Junior Subordinated Revolving Credit Facility
Related Party Transaction [Line Items]
Line of credit facilities - GM related party facility		1,000,000,000.0		1,000,000,000.0
General Motors | Revolving Credit Facility | Line of Credit
Related Party Transaction [Line Items]
Maximum borrowing capacity		14,100,000,000		$ 14,100,000,000
General Motors | Revolving Credit Facility | Line of Credit | 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
General Motors | Revolving Credit Facility | Line of Credit | 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		$ 856,000,000	$ 1,200,000,000	$ 2,500,000,000	$ 2,900,000,000