Derivative Instruments and Hedging Activities (Derivative Instruments Designated as Fair Value Hedges, Gain (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Interest and fees on finance receivables and loans	$ 1,582	$ 1,742
Interest and dividends on investment securities and other earning assets	131	226
Interest on deposits	306	592
Interest on long-term debt	250	348
Earnings on cash flow hedges to be recognized within twelve months	46
Designated as hedging instrument | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0
Derivative, Gain (Loss) on Derivative, Net	22	3
Designated as hedging instrument | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0
Derivative, Gain (Loss) on Derivative, Net	0	0
Designated as hedging instrument | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0
Derivative, Gain (Loss) on Derivative, Net	(1)	(3)
Designated as hedging instrument | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0
Derivative, Gain (Loss) on Derivative, Net	0	0
Designated as hedging instrument | Unsecured debt | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Unsecured debt | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Unsecured debt | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Unsecured debt | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	139	(170)
Change in unrealized gain (loss) on fair value hedging instruments	(139)	170
Designated as hedging instrument | Available-for-sale securities | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Available-for-sale securities | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	(13)	41
Change in unrealized gain (loss) on fair value hedging instruments	13	(41)
Designated as hedging instrument | Available-for-sale securities | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Available-for-sale securities | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Fixed-rate automotive loans | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	(39)	248
Change in unrealized gain (loss) on fair value hedging instruments	39	(248)
Designated as hedging instrument | Fixed-rate automotive loans | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Fixed-rate automotive loans | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Fixed-rate automotive loans | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Deposit liabilities | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0
Designated as hedging instrument | Deposit liabilities | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0
Designated as hedging instrument | Deposit liabilities | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	(1)	(3)
Designated as hedging instrument | Deposit liabilities | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0
Designated as hedging instrument | Variable-rate commercial loans | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	22	3
Designated as hedging instrument | Variable-rate commercial loans | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0
Designated as hedging instrument | Variable-rate commercial loans | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0
Designated as hedging instrument | Variable-rate commercial loans | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	$ 0	$ 0