INVESTMENT AND MORTGAGE-BACKED SECURITIES - Major Categories of Securities Owned (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Feb. 01, 2021
Securities, available-for-sale [Abstract]
Amortized Cost	$ 1,250,962,000		$ 1,250,962,000		$ 520,261,000
Gross Unrealized Gains	18,165,000		18,165,000		18,798,000
Gross Unrealized Losses	(7,339,000)		(7,339,000)		(198,000)
Fair Value	1,261,788,000		1,261,788,000		538,861,000
Available for sale securities	863,239,000		863,239,000		426,979,000
Acquired securities available-for-sale on the Merger Date						$ 652,000,000.0
Securities pledged as collateral	$ 505,500,000		$ 505,500,000		$ 99,400,000
Number of holdings of securities of any one issuer, other than the U.S. Government and its agencies, in an amount greater than 10% of stockholders equity	0		0		0
Threshold for disclosure percentage	10.00%				10.00%
Sales of Available-for-sale Securities:
Proceeds			$ 138,077,000	$ 62,797,000
Proceeds:
Marketable equity securities			0	273,000
Net gain (loss) on equity securities		$ 436,000	131,000	(36,000)
Marketable Equity Securities [Member]
Proceeds:
Marketable equity securities	$ 0	136,000	6,101,000	273,000
Net gain (loss) on equity securities	0	436,000	131,000	(36,000)
Agency Notes
Securities, available-for-sale [Abstract]
Amortized Cost	82,474,000		82,474,000		$ 47,500,000
Gross Unrealized Gains					12,000
Gross Unrealized Losses	(1,439,000)		(1,439,000)		(91,000)
Fair Value	81,035,000		81,035,000		47,421,000
Sales of Available-for-sale Securities:
Proceeds		273,000		273,000
Treasury Securities
Securities, available-for-sale [Abstract]
Amortized Cost	167,984,000		167,984,000
Gross Unrealized Gains	114,000		114,000
Gross Unrealized Losses	(100,000)		(100,000)
Fair Value	167,998,000		167,998,000
Corporate Securities
Securities, available-for-sale [Abstract]
Amortized Cost	103,740,000		103,740,000		62,021,000
Gross Unrealized Gains	4,380,000		4,380,000		2,440,000
Gross Unrealized Losses	(56,000)		(56,000)
Fair Value	108,064,000		108,064,000		64,461,000
Sales of Available-for-sale Securities:
Proceeds	3,519,000	25,403,000	50,273,000	25,403,000
Gross gains	20,000	1,344,000	729,000	1,344,000
Tax expense on gain	6,000	423,000	232,000	423,000
Gross losses			41,000
Tax benefit on loss			13,000
Pass-through MBS issued by GSEs
Securities, available-for-sale [Abstract]
Amortized Cost	326,786,000		326,786,000		135,842,000
Gross Unrealized Gains	6,908,000		6,908,000		7,672,000
Gross Unrealized Losses	(1,945,000)		(1,945,000)		(31,000)
Fair Value	331,749,000		331,749,000		143,483,000
Sales of Available-for-sale Securities:
Proceeds		33,195,000	26,823,000	33,195,000
Gross gains		1,790,000	187,000	1,790,000
Tax expense on gain		563,000	59,000	563,000
Gross losses	0	0	35,000	0
Tax benefit on loss	0	0	11,000	0
Agency CMOs [Member]
Securities, available-for-sale [Abstract]
Amortized Cost	528,649,000		528,649,000		274,898,000
Gross Unrealized Gains	6,532,000		6,532,000		8,674,000
Gross Unrealized Losses	(3,691,000)		(3,691,000)		(76,000)
Fair Value	531,490,000		531,490,000		$ 283,496,000
Sales of Available-for-sale Securities:
Proceeds			41,324,000	4,199,000
Gross gains			268,000	8,000
Tax expense on gain	0	0	85,000	3,000
Gross losses	0	0	44,000	0
Tax benefit on loss	0	0	14,000	0
State and municipal obligations
Securities, available-for-sale [Abstract]
Amortized Cost	41,329,000		41,329,000
Gross Unrealized Gains	231,000		231,000
Gross Unrealized Losses	(108,000)		(108,000)
Fair Value	41,452,000		41,452,000
Sales of Available-for-sale Securities:
Proceeds	0	0	19,657,000	0
Gross gains	0	0	143,000	0
Tax expense on gain	$ 0	$ 0	$ 45,000	$ 0