BORROWINGS - Additional Information (Details)	3 Months Ended			9 Months Ended
	Sep. 30, 2023 USD ($) trust	Jun. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) trust	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)	Dec. 09, 2022 USD ($)	Jan. 22, 2022 USD ($)
Debt Instrument [Line Items]
Short-term advances	$ 600,000,000			$ 600,000,000		$ 930,000,000
Short-term advances, percentage bearing fixed interest rate	36.00%			36.00%		100.00%
Aggregate principal amount	$ 620,000,000			$ 620,000,000
Proceeds from debt, net of issuance costs				613,000,000
Repayments of debt				351,000,000	$ 223,000,000
Other subordinated debt	267,000,000			267,000,000		$ 303,000,000
Subordinated notes	80,000,000			80,000,000		70,000,000
Long-term borrowings	$ 1,968,000,000			$ 1,968,000,000		1,093,000,000
Number of unconsolidated subsidiary trusts | trust	4			4
SOFR | Trust Preferred Securities
Debt Instrument [Line Items]
Tenor spread adjustment				0.0026
Howard
Debt Instrument [Line Items]
Other subordinated debt						25,000,000
Subordinated notes								$ 5,000,000
Union
Debt Instrument [Line Items]
Other subordinated debt							$ 31,000,000
Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount	$ 350,000,000			$ 350,000,000
Proceeds from debt, net of issuance costs				347,000,000
Other Subordinated Debt
Debt Instrument [Line Items]
Aggregate principal amount	270,000,000			270,000,000
Proceeds from debt, net of issuance costs				266,000,000
Repayments of subordinated debt	13,600,000
Other Subordinated Debt | Howard
Debt Instrument [Line Items]
Repayments of subordinated debt	1,000,000
Other Subordinated Debt | Union
Debt Instrument [Line Items]
Proceeds from subordinated debt		$ 6,000,000
Repayments of subordinated debt		$ 15,000,000
FHLB
Debt Instrument [Line Items]
Credit available with FHLB	10,600,000,000			10,600,000,000
Credit with FHLB utilized	$ 2,900,000,000			$ 2,900,000,000
Long-term borrowings			$ 0		0	0
FHLB | Minimum
Debt Instrument [Line Items]
Effective interest rates	4.23%			4.23%
FHLB | Maximum
Debt Instrument [Line Items]
Effective interest rates	4.88%			4.88%
FHLB | Letter of Credit
Debt Instrument [Line Items]
Credit with FHLB utilized	$ 700,000,000			$ 700,000,000
Other Wholesale Credit
Debt Instrument [Line Items]
Remaining borrowing capacity	8,600,000,000			8,600,000,000
5.150% Senior Notes due August 25, 2025 | Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount	$ 350,000,000		$ 350,000,000	$ 350,000,000	$ 350,000,000
Interest rate	5.15%		5.15%	5.15%	5.15%
Proceeds from debt, net of issuance costs			$ 347,400,000	$ 347,000,000
2.200% Senior Notes due February 2023 | Senior Notes
Debt Instrument [Line Items]
Interest rate			2.20%		2.20%
Repayments of debt			$ 300,000,000
FHLB
Debt Instrument [Line Items]
Short-term advances, overnight maturities, percentage	15.90%			15.90%
Short-term advances						$ 0