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, 2016	Dec. 31, 2015	Mar. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		$ 7,185,999	$ 7,314,504	$ 4,105,399
Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		3,133,277	3,093,160	1,564,224
Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		307,019	472,976	208,687
Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		632,130	574,370	430,963
Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		1,224,915	1,207,944	776,579
Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		340,350	376,531	208,635
One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		910,719	952,633	543,004
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		637,589	636,890	$ 373,307
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	7,044,771	7,148,698
Pass (Risk Ratings 1-5) [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	3,074,115	3,022,281
Pass (Risk Ratings 1-5) [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	303,562	468,467
Pass (Risk Ratings 1-5) [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	617,073	558,425
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	1,182,263	1,167,933
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		168,100	150,000
Pass (Risk Ratings 1-5) [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	330,929	354,760
Pass (Risk Ratings 1-5) [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	902,335	943,098
Pass (Risk Ratings 1-5) [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	634,494	633,734
Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		51,813	77,632
Special mention [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		19,845	30,928
Special mention [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		596	138
Special mention [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		3,232	2,386
Special mention [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		25,152	25,286
Special mention [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		1,885	17,526
Special mention [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		903	1,346
Special mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		200	22
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		89,406	88,164
Substandard [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		39,317	39,951
Substandard [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		2,861	4,371
Substandard [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		11,825	13,559
Substandard [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		17,500	14,725
Substandard [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		7,536	4,245
Substandard [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		7,481	8,189
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		2,886	3,124
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		9	10
Doubtful [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		9	10
Loss [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Performing loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		7,117,113	7,240,682
Performing loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		3,090,836	3,048,424
Performing loans [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		305,337	470,982
Performing loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		625,955	566,460
Performing loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		1,216,322	1,198,475
Performing loans [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		338,256	374,305
Performing loans [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		904,590	945,968
Performing loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		635,817	636,068
Nonperforming loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	15,615	15,222
Nonperforming loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	4,145	3,751
Nonperforming loans [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	0	0
Nonperforming loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	2,250	2,260
Nonperforming loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	1,557	2,167
Nonperforming loans [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	663	697
Nonperforming loans [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	5,843	5,599
Nonperforming loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	1,157	748
Purchased credit-impaired loans [Member] | Performing loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		53,271	58,600
Purchased credit-impaired loans [Member] | Performing loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		38,296	40,985
Purchased credit-impaired loans [Member] | Performing loans [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		1,682	1,994
Purchased credit-impaired loans [Member] | Performing loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		3,925	5,650
Purchased credit-impaired loans [Member] | Performing loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		7,036	7,302
Purchased credit-impaired loans [Member] | Performing loans [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		1,431	1,529
Purchased credit-impaired loans [Member] | Performing loans [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		286	1,066
Purchased credit-impaired loans [Member] | Performing loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		$ 615	$ 74

[1] The Pass category includes some performing loans that are part of homogenous pools which are not individually risk-rated. This includes all consumer loans, all one- to four-family residential loans and, as of March 31, 2016 and December 31, 2015, in the commercial business category, $168.1 million and $150.0 million, respectively, of credit-scored small business loans. As loans in these pools become non-performing, they are individually risk-rated.

[2] Non-performing loans include non-accrual loans and loans past due greater than 90 days and on accrual status.