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Fair Value Measurement - Quantitative Information about Level 3 Fair Value Measurements (Detail)
$ in Millions
Mar. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Securities available for sale (amortized cost of $78.6 billion and allowance for credit losses of $5 million as of March 31, 2020) $ 81,423 $ 79,213    
Recurring        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Securities available for sale (amortized cost of $78.6 billion and allowance for credit losses of $5 million as of March 31, 2020) 81,423 79,213    
Recurring | Level 3        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Securities available for sale (amortized cost of $78.6 billion and allowance for credit losses of $5 million as of March 31, 2020) 409 442    
Retained interests in securitizations 59 66    
Net derivative assets (liabilities) 66 26 $ 6 $ (10)
RMBS        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Securities available for sale (amortized cost of $78.6 billion and allowance for credit losses of $5 million as of March 31, 2020) 65,741 64,338    
RMBS | Recurring        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Securities available for sale (amortized cost of $78.6 billion and allowance for credit losses of $5 million as of March 31, 2020) 65,741 64,338    
RMBS | Recurring | Level 3        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Securities available for sale (amortized cost of $78.6 billion and allowance for credit losses of $5 million as of March 31, 2020) 373 429    
CMBS | Recurring        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Securities available for sale (amortized cost of $78.6 billion and allowance for credit losses of $5 million as of March 31, 2020) 10,084 9,426    
CMBS | Recurring | Level 3        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Securities available for sale (amortized cost of $78.6 billion and allowance for credit losses of $5 million as of March 31, 2020) $ 36 $ 13    
Yield | RMBS | Recurring | Level 3 | Discounted cash flows | Minimum        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Securities available-for-sale, measurement input 0.02 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 0.25 0.18    
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 0.06 0.05    
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 0.02    
Yield | CMBS | Recurring | Level 3 | Discounted cash flows | Maximum        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Securities available-for-sale, measurement input 0.03 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 0.02 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 0.03 0.04    
Voluntary prepayment rate | Recurring | Level 3 | Discounted cash flows | Maximum        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Retained interests, measurement input 0.15 0.14    
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 0.05 0.00    
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 0.18 0.18    
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 0.03 0.02    
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 0.01 0.01    
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 0.06 0.06    
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 0.70 0.74    
Loss severity | Recurring | Level 3 | Discounted cash flows | Maximum        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Retained interests, measurement input 0.75 0.88    
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 0.95 0.95    
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 0.67 0.67    
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 33 months 35 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 52 months 51 months    
Discount 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    
Discount rate | Recurring | Level 3 | Discounted cash flows | Maximum        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Retained interests, measurement input 0.11 0.10    
Swap rates | Recurring | Level 3 | Discounted cash flows | Minimum        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Net derivative assets (liabilities), measurement input 0.01 0.02    
Swap rates | Recurring | Level 3 | Discounted cash flows | Maximum        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Net derivative assets (liabilities), measurement input 0.01 0.02    
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 0.01 0.02