LOANS HELD FOR INVESTMENT, NET - Deferral Loans (Details) $ in Thousands		Jun. 30, 2021 USD ($) loan	Dec. 31, 2020 USD ($)
CARES Act Loan [Abstract]
Balance		$ 9,546,631	$ 5,622,044
COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		25
Balance	[1]	$ 44,461
% of portfolio		0.50%
One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		13
Balance	[1]	$ 12,658
% of portfolio		1.80%
Commercial Real Estate and Commercial Mixed-Use [Member] | COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		3
Balance	[1]	$ 18,343
% of portfolio		0.50%
Real estate
CARES Act Loan [Abstract]
Balance		$ 8,179,487	4,978,195
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member]
CARES Act Loan [Abstract]
Balance		704,489	184,989
Real estate | Multifamily Residential and Residential Mixed-Use [Member]
CARES Act Loan [Abstract]
Balance		3,503,205	2,758,743
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member]
CARES Act Loan [Abstract]
Balance		3,681,331	1,878,167
Real estate | ADC [Member]
CARES Act Loan [Abstract]
Balance		290,462	156,296
Commercial and industrial ("C&I") loans
CARES Act Loan [Abstract]
Balance		$ 1,343,869	641,533
Commercial and industrial ("C&I") loans | COVID-19 [Member]
CARES Act Loan [Abstract]
Number of loans | loan		9
Balance	[1]	$ 13,460
% of portfolio		1.00%
Commercial and industrial ("C&I"), including SBA and PPP loans
CARES Act Loan [Abstract]
Balance		$ 1,343,869	$ 641,533

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