Business Combinations - Narrative (Details)		3 Months Ended
	Jan. 03, 2022 USD ($) $ / shares shares	Mar. 31, 2022 USD ($) $ / shares shares	Mar. 31, 2021 USD ($)	Dec. 31, 2021 $ / shares
Business Acquisition [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares		$ 0.01		$ 0.01
Preferred stock, non-callable period, extended term	5 years
Gain on acquisition		$ 431,000,000	$ 0
Pro forma adjustment		271,000,000	$ 147,000,000
CIT Group Inc.
Business Acquisition [Line Items]
Business combination, number of fractional shares purchased (in shares) | shares	8,800
Business combination, payment to purchase fractional shares	$ 7,000,000
Consideration for stock-based compensation	81,000,000
Gain on acquisition	431,000,000
Initial PCD ACL	$ 284,000,000
Business combination, pro forma information, earnings or loss of acquiree since acquisition date, actual		330,000,000
CIT Group Inc. | Provision for Credit Losses
Business Acquisition [Line Items]
Pro forma adjustment		513,000,000
CIT Group Inc. | Acquisition-related Costs
Business Acquisition [Line Items]
Pro forma adjustment		135,000,000
CIT Group Inc. | Interest income
Business Acquisition [Line Items]
Business combination, pro forma information, revenue of acquiree since acquisition date, actual		351,000,000
CIT Group Inc. | Noninterest income
Business Acquisition [Line Items]
Business combination, pro forma information, revenue of acquiree since acquisition date, actual		$ 297,000,000
Class A Common Stock
Business Acquisition [Line Items]
Common stock, par value (in dollars per share) | $ / shares	$ 1.00	$ 1		$ 1
Class A Common Stock | CIT Group Inc.
Business Acquisition [Line Items]
Common stock, terms of conversion, conversion ratio	0.062
Business acquisition, equity interest Issued or issuable (in shares) | shares	6,140,010
Sale of stock (in dollars per share) | $ / shares	$ 859.76
Equity consideration	$ 5,279,000,000
Series A Preferred Stock
Business Acquisition [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares		$ 0.01
Preferred stock, dividend rate		5.375%
Preferred stock, shares outstanding (in shares) | shares		345,000
Liquidation preference per share (in dollars per share) | $ / shares		$ 1,000
Total Liquidation Preference		$ 345,000,000
Series B Preferred Stock
Business Acquisition [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares		$ 0.01
Preferred stock, dividend rate		5.80%
Preferred stock, shares outstanding (in shares) | shares	325,000	325,000
Liquidation preference per share (in dollars per share) | $ / shares	$ 1,000	$ 1,000
Total Liquidation Preference	$ 325,000,000	$ 325,000,000
Series C Preferred Stock
Business Acquisition [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares		$ 0.01
Preferred stock, dividend rate		5.625%
Preferred stock, shares outstanding (in shares) | shares	8,000,000	8,000,000
Liquidation preference per share (in dollars per share) | $ / shares	$ 25	$ 25
Total Liquidation Preference	$ 200,000,000	$ 200,000,000
Preferred Stock | CIT Group Inc.
Business Acquisition [Line Items]
Equity consideration	$ 541,000,000
CIT Group Inc.
Business Acquisition [Line Items]
Common stock, par value (in dollars per share) | $ / shares	$ 0.01
CIT Group Inc. | Series A Preferred Stock
Business Acquisition [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.01
CIT Group Inc. | Series B Preferred Stock
Business Acquisition [Line Items]
Common stock, terms of conversion, conversion ratio	1
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.01
Preferred stock, dividend rate	5.625%
CIT Group Inc. | Series C Preferred Stock
Business Acquisition [Line Items]
Common stock, terms of conversion, conversion ratio	1