LOANS HELD FOR INVESTMENT, NET - Deferral Loans (Details) $ in Thousands		Sep. 30, 2021 USD ($) loan	Dec. 31, 2020 USD ($)
CARES Act Loan [Abstract]
Balance		$ 9,284,871	$ 5,622,044
COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		17
Balance	[1]	$ 26,603
% of portfolio		0.30%
One-to-four family residential and cooperative/condominium apartment
CARES Act Loan [Abstract]
Balance		$ 683,665	184,989
One-to-four family residential and cooperative/condominium apartment | COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		10
Balance	[1]	$ 9,255
% of portfolio		1.40%
Commercial real estate ("CRE")
CARES Act Loan [Abstract]
Balance		$ 3,814,437	1,878,167
Commercial real estate ("CRE") | COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		1
Balance	[1]	$ 3,487
% of portfolio		0.10%
Commercial and Industrial ("C&I") Loans
CARES Act Loan [Abstract]
Balance		$ 1,012,415	$ 641,533
Commercial and Industrial ("C&I") Loans | COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		6
Balance	[1]	$ 13,861
% of portfolio		1.40%

[1] Amount excludes net deferred costs due to immateriality.