INVESTMENT SECURITIES - Allowance for credit losses to HTM securities (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Allowance for credit losses:
Beginning balance		$ 0
Impact of ASU 2016-13 adoption(1)	$ 246,256	246,256	$ 242,242	$ 240,864	$ 234,862	$ 228,024	$ 219,041
Total ending allowance balance	71	71
Corporate bonds
Allowance for credit losses:
Beginning balance	71	0
Provision for credit losses	0	0
Securities charged-off (recoveries)	0	0
Total ending allowance balance	71	71
Corporate bonds | Cumulative change in accounting principle
Allowance for credit losses:
Beginning balance		71
Corporate CMO
Allowance for credit losses:
Beginning balance	0	0
Provision for credit losses	0	0
Securities charged-off (recoveries)	0	0
Total ending allowance balance	$ 0	0
Corporate CMO | Cumulative change in accounting principle
Allowance for credit losses:
Beginning balance		$ 0