LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Risk-Rated and Non-Risk Rated Loans by Grade and Other Characteristic) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	$ 9,285,744	$ 9,305,357	$ 8,692,657
Commercial real estate - owner occupied
Financing Receivable, Credit Quality Indicator [Line Items]
2020	86,515
2019	212,466
2018	173,276
2017	141,737
2016	110,472
Prior	291,696
Revolving Loans	7,927
Total Loans	1,024,089
Loans receivable	1,024,089	1,580,650
Commercial real estate - investment properties
Financing Receivable, Credit Quality Indicator [Line Items]
2020	42,548
2019	303,422
2018	350,346
2017	309,865
2016	328,681
Prior	642,391
Revolving Loans	30,284
Total Loans	2,007,537
Loans receivable	2,007,537	2,309,221
Multifamily real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	21,907
2019	72,099
2018	45,457
2017	102,668
2016	47,554
Prior	108,743
Revolving Loans	1,778
Total Loans	400,206
Loans receivable	400,206	473,152	387,142
Commercial construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	17,290
2019	113,200
2018	61,732
2017	9,326
2016	2,329
Prior	1,599
Revolving Loans	0
Total Loans	205,476
Loans receivable	205,476	210,668
Multifamily construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	39,395
2019	115,501
2018	76,925
2017	12,945
2016	0
Prior	0
Revolving Loans	5,644
Total Loans	250,410
Loans receivable	250,410	233,610
One- to four- family construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	145,711
2019	372,014
2018	11,536
2017	0
2016	7
Prior	0
Revolving Loans	5,688
Total Loans	534,956
Loans receivable	534,956	544,308
Land and land development
Financing Receivable, Credit Quality Indicator [Line Items]
2020	36,452
2019	126,935
2018	44,296
2017	10,708
2016	7,275
Prior	6,580
Revolving Loans	260
Total Loans	232,506
Commercial business
Financing Receivable, Credit Quality Indicator [Line Items]
2020	77,653
2019	288,311
2018	241,005
2017	103,908
2016	57,894
Prior	95,077
Revolving Loans	493,969
Total Loans	1,357,817
Loans receivable	1,357,817	1,693,824	1,524,298
Agricultural business, including secured by farmland
Financing Receivable, Credit Quality Indicator [Line Items]
2020	18,553
2019	71,160
2018	41,908
2017	27,945
2016	28,141
Prior	37,258
Revolving Loans	105,292
Total Loans	330,257
Loans receivable	330,257	370,549	373,322
Small balance CRE
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3,076
2019	72,601
2018	91,837
2017	76,082
2016	80,463
Prior	266,535
Revolving Loans	1,189
Total Loans	591,783
Loans receivable	591,783
Small business scored
Financing Receivable, Credit Quality Indicator [Line Items]
2020	49,439
2019	177,749
2018	162,371
2017	117,773
2016	61,181
Prior	86,373
Revolving Loans	152,653
Total Loans	807,539
Loans receivable	807,539
One- to four- family residential
Financing Receivable, Credit Quality Indicator [Line Items]
2020	11,236
2019	123,181
2018	142,786
2017	157,706
2016	77,516
Prior	364,161
Revolving Loans	4,801
Total Loans	881,387
Loans receivable	881,387	945,622	$ 967,581
Consumer—home equity revolving lines of credit
Financing Receivable, Credit Quality Indicator [Line Items]
2020	14,227
2019	2,053
2018	1,774
2017	2,657
2016	1,520
Prior	4,279
Revolving Loans	495,108
Total Loans	521,618
Consumer—other
Financing Receivable, Credit Quality Indicator [Line Items]
2020	7,301
2019	22,207
2018	22,188
2017	18,666
2016	13,034
Prior	26,421
Revolving Loans	30,346
Total Loans	140,163
Loans receivable		211,815
Land and land development - residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	232,506	154,688
Land and land development - commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		26,290
Consumer secured by one- to four-family
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	521,618	550,960
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		9,130,662
Pass | Commercial real estate - owner occupied
Financing Receivable, Credit Quality Indicator [Line Items]
2020	86,515
2019	209,528
2018	171,907
2017	133,579
2016	109,177
Prior	280,979
Revolving Loans	7,927
Total Loans	999,612
Loans receivable		1,546,649
Pass | Commercial real estate - investment properties
Financing Receivable, Credit Quality Indicator [Line Items]
2020	42,548
2019	296,128
2018	347,941
2017	309,865
2016	328,239
Prior	631,602
Revolving Loans	30,284
Total Loans	1,986,607
Loans receivable		2,288,785
Pass | Multifamily real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	21,907
2019	72,099
2018	45,457
2017	102,668
2016	47,554
Prior	108,743
Revolving Loans	1,778
Total Loans	400,206
Loans receivable		472,856
Pass | Commercial construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	17,290
2019	101,679
2018	55,535
2017	9,326
2016	2,231
Prior	1,599
Revolving Loans	0
Total Loans	187,660
Loans receivable		198,986
Pass | Multifamily construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	39,395
2019	115,501
2018	76,925
2017	12,945
2016	0
Prior	0
Revolving Loans	5,644
Total Loans	250,410
Loans receivable		233,610
Pass | One- to four- family construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	143,457
2019	357,179
2018	11,536
2017	0
2016	7
Prior	0
Revolving Loans	5,058
Total Loans	517,237
Loans receivable		530,307
Pass | Land and land development
Financing Receivable, Credit Quality Indicator [Line Items]
2020	36,452
2019	126,901
2018	44,296
2017	10,708
2016	7,275
Prior	6,580
Revolving Loans	260
Total Loans	232,472
Pass | Commercial business
Financing Receivable, Credit Quality Indicator [Line Items]
2020	74,684
2019	284,798
2018	228,436
2017	98,502
2016	52,755
Prior	89,037
Revolving Loans	466,294
Total Loans	1,294,506
Loans receivable		1,627,170
Pass | Agricultural business, including secured by farmland
Financing Receivable, Credit Quality Indicator [Line Items]
2020	17,928
2019	64,309
2018	35,934
2017	26,835
2016	27,403
Prior	35,133
Revolving Loans	102,965
Total Loans	310,507
Loans receivable		352,408
Pass | Small business scored
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		764,600
Pass | One- to four- family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		940,424
Pass | Consumer—other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		211,475
Pass | Land and land development - residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		154,348
Pass | Land and land development - commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		26,256
Pass | Consumer secured by one- to four-family
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		547,388
Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		61,189
Special Mention | Commercial real estate - owner occupied
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	2,438
2018	1,369
2017	2,353
2016	0
Prior	103
Revolving Loans	0
Total Loans	6,263
Loans receivable		4,198
Special Mention | Commercial real estate - investment properties
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	3,360
2018	0
2017	0
2016	0
Prior	976
Revolving Loans	0
Total Loans	4,336
Loans receivable		2,193
Special Mention | Multifamily real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Special Mention | Commercial construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	6,197
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	6,197
Loans receivable		0
Special Mention | Multifamily construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Special Mention | One- to four- family construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,254
2019	13,540
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	630
Total Loans	16,424
Loans receivable		12,534
Special Mention | Land and land development
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Special Mention | Commercial business
Financing Receivable, Credit Quality Indicator [Line Items]
2020	65
2019	1,314
2018	5,021
2017	1,472
2016	4,846
Prior	5,091
Revolving Loans	8,635
Total Loans	26,444
Loans receivable		31,012
Special Mention | Agricultural business, including secured by farmland
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	1,328
2018	0
2017	919
2016	676
Prior	1,418
Revolving Loans	0
Total Loans	4,341
Loans receivable		10,840
Special Mention | One- to four- family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		409
Special Mention | Consumer—other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		3
Special Mention | Land and land development - residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Special Mention | Land and land development - commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Special Mention | Consumer secured by one- to four-family
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		113,448
Substandard | Commercial real estate - owner occupied
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	500
2018	0
2017	5,805
2016	1,295
Prior	10,614
Revolving Loans	0
Total Loans	18,214
Loans receivable		29,803
Substandard | Commercial real estate - investment properties
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	3,934
2018	2,405
2017	0
2016	442
Prior	9,813
Revolving Loans	0
Total Loans	16,594
Loans receivable		18,243
Substandard | Multifamily real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		296
Substandard | Commercial construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	11,521
2018	0
2017	0
2016	98
Prior	0
Revolving Loans	0
Total Loans	11,619
Loans receivable		11,682
Substandard | Multifamily construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Substandard | One- to four- family construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	1,295
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	1,295
Loans receivable		1,467
Substandard | Land and land development
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	34
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	34
Substandard | Commercial business
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,904
2019	2,199
2018	7,548
2017	3,934
2016	293
Prior	949
Revolving Loans	19,040
Total Loans	36,867
Loans receivable		35,584
Substandard | Agricultural business, including secured by farmland
Financing Receivable, Credit Quality Indicator [Line Items]
2020	625
2019	5,523
2018	5,974
2017	191
2016	62
Prior	707
Revolving Loans	2,327
Total Loans	15,409
Loans receivable		7,301
Substandard | One- to four- family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		4,789
Substandard | Consumer—other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		337
Substandard | Land and land development - residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		340
Substandard | Land and land development - commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		34
Substandard | Consumer secured by one- to four-family
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		3,572
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		58
Doubtful | Commercial real estate - owner occupied
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Doubtful | Commercial real estate - investment properties
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Doubtful | Multifamily real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Doubtful | Commercial construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Doubtful | Multifamily construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Doubtful | One- to four- family construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Doubtful | Land and land development
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Doubtful | Commercial business
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		58
Doubtful | Agricultural business, including secured by farmland
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Doubtful | One- to four- family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Doubtful | Consumer—other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Doubtful | Land and land development - residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Doubtful | Land and land development - commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Doubtful | Consumer secured by one- to four-family
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Loss | Commercial real estate - owner occupied
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Loss | Commercial real estate - investment properties
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Loss | Multifamily real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Loss | Commercial construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Loss | Multifamily construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Loss | One- to four- family construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Loss | Land and land development
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loss | Commercial business
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Loss | Agricultural business, including secured by farmland
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Total Loans	0
Loans receivable		0
Loss | One- to four- family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Loss | Consumer—other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Loss | Land and land development - residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Loss | Land and land development - commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		0
Loss | Consumer secured by one- to four-family
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable		$ 0
Current | Small balance CRE
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3,076
2019	72,221
2018	91,455
2017	75,742
2016	80,463
Prior	265,178
Revolving Loans	1,189
Total Loans	589,324
Current | Small business scored
Financing Receivable, Credit Quality Indicator [Line Items]
2020	49,408
2019	176,863
2018	161,438
2017	115,884
2016	60,448
Prior	85,896
Revolving Loans	151,905
Total Loans	801,842
Current | One- to four- family residential
Financing Receivable, Credit Quality Indicator [Line Items]
2020	10,298
2019	119,074
2018	141,540
2017	156,046
2016	77,252
Prior	357,521
Revolving Loans	4,801
Total Loans	866,532
Current | Consumer—home equity revolving lines of credit
Financing Receivable, Credit Quality Indicator [Line Items]
2020	14,227
2019	2,053
2018	1,774
2017	2,137
2016	1,223
Prior	3,840
Revolving Loans	493,903
Total Loans	519,157
Current | Consumer—other
Financing Receivable, Credit Quality Indicator [Line Items]
2020	7,199
2019	22,156
2018	21,985
2017	18,571
2016	12,885
Prior	26,316
Revolving Loans	30,214
Total Loans	139,326
30-59 Days Past Due | Small balance CRE
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	380
2018	382
2017	0
2016	0
Prior	738
Revolving Loans	0
Total Loans	1,500
30-59 Days Past Due | Small business scored
Financing Receivable, Credit Quality Indicator [Line Items]
2020	31
2019	459
2018	215
2017	977
2016	68
Prior	325
Revolving Loans	466
Total Loans	2,541
30-59 Days Past Due | One- to four- family residential
Financing Receivable, Credit Quality Indicator [Line Items]
2020	559
2019	4,107
2018	639
2017	1,615
2016	264
Prior	5,215
Revolving Loans	0
Total Loans	12,399
30-59 Days Past Due | Consumer—home equity revolving lines of credit
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	14
Revolving Loans	664
Total Loans	678
30-59 Days Past Due | Consumer—other
Financing Receivable, Credit Quality Indicator [Line Items]
2020	102
2019	21
2018	174
2017	78
2016	100
Prior	95
Revolving Loans	78
Total Loans	648
60-89 Days Past Due | Small balance CRE
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	396
Revolving Loans	0
Total Loans	396
60-89 Days Past Due | Small business scored
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	184
2018	150
2017	197
2016	55
Prior	0
Revolving Loans	5
Total Loans	591
60-89 Days Past Due | One- to four- family residential
Financing Receivable, Credit Quality Indicator [Line Items]
2020	21
2019	0
2018	0
2017	0
2016	0
Prior	82
Revolving Loans	0
Total Loans	103
60-89 Days Past Due | Consumer—home equity revolving lines of credit
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	267
Revolving Loans	0
Total Loans	267
60-89 Days Past Due | Consumer—other
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	30
2018	29
2017	17
2016	0
Prior	10
Revolving Loans	54
Total Loans	140
90 Days Past Due | Small balance CRE
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	340
2016	0
Prior	223
Revolving Loans	0
Total Loans	563
90 Days Past Due | Small business scored
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	243
2018	568
2017	715
2016	610
Prior	152
Revolving Loans	277
Total Loans	2,565
90 Days Past Due | One- to four- family residential
Financing Receivable, Credit Quality Indicator [Line Items]
2020	358
2019	0
2018	607
2017	45
2016	0
Prior	1,343
Revolving Loans	0
Total Loans	2,353
90 Days Past Due | Consumer—home equity revolving lines of credit
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	520
2016	297
Prior	158
Revolving Loans	541
Total Loans	1,516
90 Days Past Due | Consumer—other
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	49
Prior	0
Revolving Loans	0
Total Loans	$ 49