LOANS HELD FOR INVESTMENT, NET - Deferral Loans (Details) $ in Thousands		Mar. 31, 2021 USD ($) loan	Dec. 31, 2020 USD ($)
CARES Act Loan [Abstract]
Balance		$ 10,506,098	$ 5,622,044
COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		34
Balance	[1]	$ 66,654
% of portfolio		0.60%
One-to-four family residential and cooperative/condominium apartment | COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		15
Balance	[1]	$ 15,489
% of portfolio		2.20%
Commercial real estate ("CRE") | COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		8
Balance	[1]	$ 24,174
% of portfolio		0.70%
Acquisition, development, and construction ("ADC") | COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		1
Balance	[1]	$ 13,500
% of portfolio		5.30%
Real estate
CARES Act Loan [Abstract]
Balance		$ 8,149,079	4,978,195
Real estate | One-to-four family residential and cooperative/condominium apartment
CARES Act Loan [Abstract]
Balance		696,415	184,989
Real estate | Multifamily residential and residential mixed-use
CARES Act Loan [Abstract]
Balance		3,567,207	2,758,743
Real estate | Commercial real estate ("CRE")
CARES Act Loan [Abstract]
Balance		3,631,287	1,878,167
Real estate | Acquisition, development, and construction ("ADC")
CARES Act Loan [Abstract]
Balance		254,170	156,296
Commercial and industrial ("C&I")
CARES Act Loan [Abstract]
Balance		2,332,610	641,533
Commercial and industrial ("C&I"), including SBA and PPP loans
CARES Act Loan [Abstract]
Balance		$ 2,332,610	$ 641,533
Commercial and industrial ("C&I"), including SBA and PPP loans | COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		10
Balance	[1]	$ 13,491
% of portfolio		0.60%

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