INVESTMENT AND MORTGAGE-BACKED SECURITIES - Securities Available-for-sale Major Categories (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Securities available-for-sale:
Amortized Cost	$ 1,706,892	$ 520,261
Gross Unrealized Gains	16,080	18,798
Gross Unrealized Losses	(13,909)	(198)
Fair Value	1,709,063	538,861
Agency Notes
Securities available-for-sale:
Amortized Cost	82,475	47,500
Gross Unrealized Gains		12
Gross Unrealized Losses	(1,457)	(91)
Fair Value	81,018	47,421
Treasury Securities
Securities available-for-sale:
Amortized Cost	248,173
Gross Unrealized Gains	5
Gross Unrealized Losses	(482)
Fair Value	247,696
Corporate Securities
Securities available-for-sale:
Amortized Cost	122,476	62,021
Gross Unrealized Gains	4,758	2,440
Gross Unrealized Losses	(401)
Fair Value	126,833	64,461
Pass-through MBS issued by GSEs
Securities available-for-sale:
Amortized Cost	622,225	135,842
Gross Unrealized Gains	6,005	7,672
Gross Unrealized Losses	(6,273)	(31)
Fair Value	621,957	143,483
Agency Collateralized Mortgage Obligations ("CMOs")
Securities available-for-sale:
Amortized Cost	590,385	274,898
Gross Unrealized Gains	5,159	8,674
Gross Unrealized Losses	(5,118)	(76)
Fair Value	590,426	$ 283,496
State and municipal obligations.
Securities available-for-sale:
Amortized Cost	41,158
Gross Unrealized Gains	153
Gross Unrealized Losses	(178)
Fair Value	$ 41,133

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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-Topic 320
-SubTopic 10
-Section 50
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Publisher FASB
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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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