DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Notional Amounts and Gross Fair Values (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Derivatives, Fair Value [Line Items]
Notional amount	$ 14,235.0	$ 13,718.0
Derivative asset, not offset against collateral	140.0	89.0
Derivative liability, not offset against collateral	6.0	7.0
Derivative asset, not subject to master netting arrangement	5.0	7.0
Derivative liability, not subject to master netting arrangement	463.0	404.0
Fair value, asset	145.0	96.0
Fair value, liability	469.0	411.0
Interest rate contracts – designated
Derivatives, Fair Value [Line Items]
Derivative asset, not offset against collateral	0.0	0.0
Derivative liability, not offset against collateral	4.0	1.0
Interest rate swaps – not designated
Derivatives, Fair Value [Line Items]
Derivative asset, not offset against collateral	140.0	89.0
Derivative liability, not offset against collateral	2.0	6.0
Derivative asset, not subject to master netting arrangement	2.0	6.0
Derivative liability, not subject to master netting arrangement	460.0	390.0
Interest rate lock commitments – not designated
Derivatives, Fair Value [Line Items]
Derivative asset, not subject to master netting arrangement	0.0	0.0
Derivative liability, not subject to master netting arrangement	3.0	12.0
Forward delivery commitments – not designated
Derivatives, Fair Value [Line Items]
Derivative asset, not subject to master netting arrangement	3.0	1.0
Derivative liability, not subject to master netting arrangement	0.0	2.0
Credit risk contracts – not designated
Derivatives, Fair Value [Line Items]
Notional amount	529.6
Derivative asset, not subject to master netting arrangement	0.0	0.0
Derivative liability, not subject to master netting arrangement	0.0	0.0
Subject to master netting arrangements:
Derivatives, Fair Value [Line Items]
Notional amount	7,577.0	7,513.0
Subject to master netting arrangements: | Interest rate contracts – designated
Derivatives, Fair Value [Line Items]
Notional amount	2,000.0	2,180.0
Subject to master netting arrangements: | Interest rate swaps – not designated
Derivatives, Fair Value [Line Items]
Notional amount	5,577.0	5,333.0
Not subject to master netting arrangements:
Derivatives, Fair Value [Line Items]
Notional amount	6,658.0	6,205.0
Not subject to master netting arrangements: | Interest rate swaps – not designated
Derivatives, Fair Value [Line Items]
Notional amount	5,577.0	5,333.0
Not subject to master netting arrangements: | Interest rate lock commitments – not designated
Derivatives, Fair Value [Line Items]
Notional amount	234.0	163.0
Not subject to master netting arrangements: | Forward delivery commitments – not designated
Derivatives, Fair Value [Line Items]
Notional amount	198.0	203.0
Not subject to master netting arrangements: | Credit risk contracts – not designated
Derivatives, Fair Value [Line Items]
Notional amount	$ 649.0	$ 506.0