Fair Value Measurements - Assets Measured at Fair Value on Nonrecurring Basis (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	$ 5,996		$ 5,996		$ 7,009
Total assets at fair value	6,007		6,007		7,021
Net gains (losses) on loan sales	169	$ 346	564	$ 739
Loans held-for-sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	5,996		5,996		7,009
Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	415		415		119
Net gains (losses) on loan sales			(14)		(7)
Nonrecurring | Loans held-for-sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	363		363
Fair value gains (losses) on loans			(1)
Nonrecurring | Residential first mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential first mortgage loans					30
Fair value gains (losses) on loans					(1)
Nonrecurring | Commercial loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial loans	19		19		57
Fair value gains (losses) on loans			0		0
Nonrecurring | Impaired residential first mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans held-for-investment	27		27		24
Fair value gains (losses) on loans			(12)		(3)
Nonrecurring | Repossessed assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repossessed assets	6		6		8
Fair value gains (losses) on repossessed assets			(1)		(3)
Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	363		363		30
Nonrecurring | Level 2 | Loans held-for-sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	363		363
Nonrecurring | Level 2 | Residential first mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential first mortgage loans					30
Nonrecurring | Level 2 | Commercial loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial loans	0		0		0
Nonrecurring | Level 2 | Impaired residential first mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans held-for-investment	0		0		0
Nonrecurring | Level 2 | Repossessed assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repossessed assets	0		0		0
Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	52		52		89
Nonrecurring | Level 3 | Loans held-for-sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0		0
Nonrecurring | Level 3 | Residential first mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential first mortgage loans					0
Nonrecurring | Level 3 | Commercial loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial loans	19		19		57
Nonrecurring | Level 3 | Impaired residential first mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans held-for-investment	27		27		24
Nonrecurring | Level 3 | Repossessed assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repossessed assets	$ 6		$ 6		$ 8