CREDIT QUALITY ASSESSMENT - Allowance for credit loss activity (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			$ 165,367	$ 56,132	$ 56,132
Initial allowance on acquired Revere PCD loans			0	18,628	18,628
Provision/ (credit) for credit losses	$ (8,229)	$ 7,003	(47,141)	90,158	85,669
Loan charge-offs			(11,496)	(1,255)	(1,819)
Loan recoveries			1,190	906	1,012
Net charge-offs			(10,306)	(349)	(807)
Balance at period end	$ 107,920	$ 170,314	107,920	170,314	165,367
Revision of Prior Period, Reclassification, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			0	2,762	2,762
Balance at period end					0
Revision of Prior Period, Accounting Standards Update, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			$ 0	$ 2,983	2,983
Balance at period end					$ 0