Fair Value Measurement - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	$ 99,165	$ 100,445
Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	99,165	100,445
Recurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	336	439
Retained interests in securitizations	50	55
Net derivative assets (liabilities)	34	31	$ 66	$ 26
RMBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	75,622	76,703
RMBS | Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	75,622	76,703
RMBS | Recurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	326	328
CMBS | Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	11,274	11,735
CMBS | Recurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	$ 10	$ 111
Yield | RMBS | Recurring | Level 3 | Discounted cash flows | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.01	0.02
Yield | RMBS | Recurring | Level 3 | Discounted cash flows | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.19	0.12
Yield | RMBS | Recurring | Level 3 | Discounted cash flows | Weighted average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.03	0.03
Yield | CMBS | Recurring | Level 3 | Discounted cash flows | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.01	0.01
Yield | CMBS | Recurring | Level 3 | Discounted cash flows | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.02	0.03
Yield | CMBS | Recurring | Level 3 | Discounted cash flows | Weighted average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.01	0.02
Voluntary prepayment rate | Recurring | Level 3 | Discounted cash flows | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Retained interests, measurement input	0.09	0.03
Voluntary prepayment rate | Recurring | Level 3 | Discounted cash flows | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Retained interests, measurement input	0.12	0.13
Voluntary prepayment rate | RMBS | Recurring | Level 3 | Discounted cash flows | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.03	0.08
Voluntary prepayment rate | RMBS | Recurring | Level 3 | Discounted cash flows | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.15	0.15
Voluntary prepayment rate | RMBS | Recurring | Level 3 | Discounted cash flows | Weighted average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.10	0.10
Default rate | Recurring | Level 3 | Discounted cash flows | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Retained interests, measurement input	0.02	0.03
Default rate | Recurring | Level 3 | Discounted cash flows | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Retained interests, measurement input	0.04	0.03
Default rate | RMBS | Recurring | Level 3 | Discounted cash flows | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0	0
Default rate | RMBS | Recurring | Level 3 | Discounted cash flows | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.11	0.11
Default rate | RMBS | Recurring | Level 3 | Discounted cash flows | Weighted average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.02	0.02
Loss severity | Recurring | Level 3 | Discounted cash flows | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Retained interests, measurement input	0.49	0.55
Loss severity | Recurring | Level 3 | Discounted cash flows | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Retained interests, measurement input	1.51	0.70
Loss severity | RMBS | Recurring | Level 3 | Discounted cash flows | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.30	0.30
Loss severity | RMBS | Recurring | Level 3 | Discounted cash flows | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	1	1
Loss severity | RMBS | Recurring | Level 3 | Discounted cash flows | Weighted average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available-for-sale, measurement input	0.72	0.73
Life of receivables (months) | Recurring | Level 3 | Discounted cash flows | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Retained interests, measurement input, life of receivables	36 months	37 months
Life of receivables (months) | Recurring | Level 3 | Discounted cash flows | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Retained interests, measurement input, life of receivables	46 months	52 months
Discount rate | Recurring | Level 3 | Discounted cash flows | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Retained interests, measurement input	0.03	0.02
Discount rate | Recurring | Level 3 | Discounted cash flows | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Retained interests, measurement input	0.13	0.12
Swap rates | Recurring | Level 3 | Discounted cash flows | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net derivative assets (liabilities), measurement input	0.02	0.01
Swap rates | Recurring | Level 3 | Discounted cash flows | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net derivative assets (liabilities), measurement input	0.02	0.01
Swap rates | Recurring | Level 3 | Discounted cash flows | Weighted average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net derivative assets (liabilities), measurement input	0.02	0.01