Refers to the amount of income tax benefit related to loss on sale of mortgage backed securities.

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Refers to the amount of income tax expenses related to gain on sale of mortgage backed securities.

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Proceeds abstract.

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Threshold percentage which the entity uses for disclosure.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value at acquisition of acquired loans accounted for as available-for-sale debt securities with evidence of deterioration of credit quality.

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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The cash inflow associated with the aggregate amount received by the entity through sale or maturity of marketable securities (held-to-maturity or available-for-sale) during the period.

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Amount of cash inflow from sale of investment in mortgage-backed security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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INVESTMENT AND MORTGAGE-BACKED SECURITIES - Major Categories of Securities Owned (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Feb. 01, 2021
Securities, available-for-sale [Abstract]
Amortized Cost	$ 1,150,614		$ 520,261
Gross Unrealized Gains	16,904		18,798
Gross Unrealized Losses	(15,025)		(198)
Fair Value	1,152,493		538,861
Available for sale securities	846,529		426,979
Acquired securities available-for-sale on the Merger Date				$ 652,000
Securities pledged as collateral	$ 637,100		$ 99,400
Threshold for disclosure percentage	10.00%		10.00%
Sales of Available-for-sale Securities:
Proceeds	$ 134,558	$ 4,199
Proceeds:
Marketable equity securities	6,101	137
Net gain (loss) on equity securities	131	(472)
Marketable Equity Securities [Member]
Proceeds:
Marketable equity securities	6,101	137
Net gain (loss) on equity securities	131	472
Agency Notes
Securities, available-for-sale [Abstract]
Amortized Cost	72,500		$ 47,500
Gross Unrealized Gains			12
Gross Unrealized Losses	(2,930)		(91)
Fair Value	69,570		47,421
Treasury Securities
Securities, available-for-sale [Abstract]
Amortized Cost	92,547
Gross Unrealized Losses	(275)
Fair Value	92,272
Corporate Securities
Securities, available-for-sale [Abstract]
Amortized Cost	95,786		62,021
Gross Unrealized Gains	3,367		2,440
Gross Unrealized Losses	(420)
Fair Value	98,733		64,461
Sales of Available-for-sale Securities:
Proceeds	46,754	0
Gross gains	709	0
Tax expense on gain	225	0
Gross losses	41	0
Tax benefit on loss	13	0
Pass-through MBS issued by GSEs
Securities, available-for-sale [Abstract]
Amortized Cost	321,442		135,842
Gross Unrealized Gains	7,053		7,672
Gross Unrealized Losses	(4,788)		(31)
Fair Value	323,707		143,483
Sales of Available-for-sale Securities:
Proceeds	26,823
Gross gains	187
Tax expense on gain	59
Gross losses	35	0
Tax benefit on loss	11	0
Agency Collateralized Mortgage Obligations ("CMOs")
Securities, available-for-sale [Abstract]
Amortized Cost	522,841		274,898
Gross Unrealized Gains	6,305		8,674
Gross Unrealized Losses	(6,324)		(76)
Fair Value	522,822		$ 283,496
Sales of Available-for-sale Securities:
Proceeds	41,324	4,199
Gross gains	268	8
Tax expense on gain	85	3
Gross losses	44	0
Tax benefit on loss	14	0
State and municipal obligations
Securities, available-for-sale [Abstract]
Amortized Cost	45,498
Gross Unrealized Gains	179
Gross Unrealized Losses	(288)
Fair Value	45,389
Sales of Available-for-sale Securities:
Proceeds	19,657	0
Gross gains	143	0
Tax expense on gain	$ 45	$ 0