LOANS RECEIVABLE AND THE ALLOWANCE FOR CREDIT LOSSES (Allowance for Loan Losses) (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	$ 148,009		$ 156,352	$ 167,279	$ 100,559
(Recapture)/provision for credit losses	(8,850)		13,641	(24,985)	64,878
Recoveries	1,416		445	3,521	3,453
Charge-offs	(660)		(2,473)	(5,900)	(8,737)
Allowance for loan losses, Ending balance	139,915	$ 148,009	167,965	139,915	167,965
Impact of Adopting ASC 326
Financing Receivable, Allowance for Loan Losses [Roll Forward]
(Recapture)/provision for credit losses		7,812
Commercial Real Estate
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	60,349		53,166	57,791	30,591
(Recapture)/provision for credit losses	(4,057)		6,895	2,096	32,213
Recoveries	923		23	1,094	244
Charge-offs	0		(379)	(3,766)	(479)
Allowance for loan losses, Ending balance	57,215	60,349	59,705	57,215	59,705
Commercial Real Estate | Impact of Adopting ASC 326
Financing Receivable, Allowance for Loan Losses [Roll Forward]
(Recapture)/provision for credit losses		(2,864)
Multifamily Real Estate
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	5,807		3,504	3,893	4,754
(Recapture)/provision for credit losses	850		(248)	2,764	772
Recoveries	0		0	0	0
Charge-offs	0		0	0	(66)
Allowance for loan losses, Ending balance	6,657	5,807	3,256	6,657	3,256
Multifamily Real Estate | Impact of Adopting ASC 326
Financing Receivable, Allowance for Loan Losses [Roll Forward]
(Recapture)/provision for credit losses		(2,204)
Construction and Land
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	30,899		36,916	41,295	22,994
(Recapture)/provision for credit losses	(1,557)		2,561	(12,053)	13,963
Recoveries	0		0	100	105
Charge-offs	0		0	0	(100)
Allowance for loan losses, Ending balance	29,342	30,899	39,477	29,342	39,477
Construction and Land | Impact of Adopting ASC 326
Financing Receivable, Allowance for Loan Losses [Roll Forward]
(Recapture)/provision for credit losses		2,515
Commercial Business
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	30,830		33,870	35,007	23,370
(Recapture)/provision for credit losses	(3,825)		2,550	(8,390)	14,402
Recoveries	230		246	1,530	821
Charge-offs	(362)		(1,297)	(1,274)	(6,234)
Allowance for loan losses, Ending balance	26,873	30,830	35,369	26,873	35,369
Commercial Business | Impact of Adopting ASC 326
Financing Receivable, Allowance for Loan Losses [Roll Forward]
(Recapture)/provision for credit losses		3,010
Agricultural Business
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	3,256		4,517	4,914	4,120
(Recapture)/provision for credit losses	83		1,026	(1,581)	64
Recoveries	17		0	25	1,772
Charge-offs	(179)		(492)	(181)	(554)
Allowance for loan losses, Ending balance	3,177	3,256	5,051	3,177	5,051
Agricultural Business | Impact of Adopting ASC 326
Financing Receivable, Allowance for Loan Losses [Roll Forward]
(Recapture)/provision for credit losses		(351)
One- to Four-Family Residential
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	9,800		12,746	9,913	4,136
(Recapture)/provision for credit losses	(359)		100	(605)	1,470
Recoveries	19		94	152	273
Charge-offs	0		(72)	0	(136)
Allowance for loan losses, Ending balance	9,460	9,800	12,868	9,460	12,868
One- to Four-Family Residential | Impact of Adopting ASC 326
Financing Receivable, Allowance for Loan Losses [Roll Forward]
(Recapture)/provision for credit losses		7,125
Consumer
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	7,068		11,633	14,466	8,202
(Recapture)/provision for credit losses	15		757	(7,216)	1,994
Recoveries	227		82	620	238
Charge-offs	(119)		(233)	(679)	(1,168)
Allowance for loan losses, Ending balance	7,191	7,068	12,239	7,191	12,239
Consumer | Impact of Adopting ASC 326
Financing Receivable, Allowance for Loan Losses [Roll Forward]
(Recapture)/provision for credit losses		2,973
Unallocated
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	0		0	0	2,392
(Recapture)/provision for credit losses	0		0	0	0
Recoveries	0		0	0	0
Charge-offs	0		0	0	0
Allowance for loan losses, Ending balance	$ 0	0	$ 0	$ 0	$ 0
Unallocated | Impact of Adopting ASC 326
Financing Receivable, Allowance for Loan Losses [Roll Forward]
(Recapture)/provision for credit losses		$ (2,392)