LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Risk-Rate and Non-Risk Rated Loans by Grade and Other Characteristic) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	$ 7,314,504	$ 3,831,034
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	3,093,160	1,403,725
Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	472,976	167,524
Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	574,370	411,006
Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,207,944	723,964
Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	376,531	238,499
One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	952,633	537,108
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	636,890	349,208
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	7,148,698	3,734,251
Pass (Risk Ratings 1-5) [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	3,022,281	1,375,885
Pass (Risk Ratings 1-5) [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	468,467	166,712
Pass (Risk Ratings 1-5) [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	558,425	395,356
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,167,933	691,143
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	150,000	115,000
Pass (Risk Ratings 1-5) [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	354,760	234,101
Pass (Risk Ratings 1-5) [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	943,098	524,598
Pass (Risk Ratings 1-5) [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	633,734	346,456
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	77,632	32,428
Special Mention [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	30,928	3,717
Special Mention [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	138	0
Special Mention [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	2,386	0
Special Mention [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	25,286	27,453
Special Mention [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	17,526	1,055
Special Mention [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,346	63
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	22	140
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	88,164	64,344
Substandard [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	39,951	24,123
Substandard [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	4,371	812
Substandard [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	13,559	15,650
Substandard [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	14,725	5,368
Substandard [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	4,245	3,343
Substandard [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	8,189	12,447
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	3,124	2,601
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	10	11
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 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	10	11
Unlikely to be Collected Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0		$ 0
Unlikely to be Collected Financing Receivable [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0		0
Unlikely to be Collected Financing Receivable [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0		0
Unlikely to be Collected Financing Receivable [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0		0
Unlikely to be Collected Financing Receivable [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0		0
Unlikely to be Collected Financing Receivable [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0		0
Unlikely to be Collected Financing Receivable [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0		0
Unlikely to be Collected Financing Receivable [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0		$ 0
Performing loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	7,240,682	3,814,298
Performing loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	3,048,424	1,402,328
Performing loans [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	470,982	167,524
Performing loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	566,460	409,731
Performing loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,198,475	723,427
Performing loans [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	374,305	236,902
Performing loans [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	945,968	526,506
Performing loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	636,068	347,880
Nonperforming loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	15,222	16,736
Nonperforming loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	3,751	1,397
Nonperforming loans [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Nonperforming loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	2,260	1,275
Nonperforming loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	2,167	537
Nonperforming loans [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	697	1,597
Nonperforming loans [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	5,599	10,602
Nonperforming loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	748	$ 1,328
Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	58,600
Receivables Acquired with Deteriorated Credit Quality [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	40,985
Receivables Acquired with Deteriorated Credit Quality [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,994
Receivables Acquired with Deteriorated Credit Quality [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	5,650
Receivables Acquired with Deteriorated Credit Quality [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	7,302
Receivables Acquired with Deteriorated Credit Quality [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,529
Receivables Acquired with Deteriorated Credit Quality [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,066
Receivables Acquired with Deteriorated Credit Quality [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	74
Receivables Acquired with Deteriorated Credit Quality [Member] | Performing loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	58,600
Receivables Acquired with Deteriorated Credit Quality [Member] | Performing loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	40,985
Receivables Acquired with Deteriorated Credit Quality [Member] | Performing loans [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,994
Receivables Acquired with Deteriorated Credit Quality [Member] | Performing loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	5,650
Receivables Acquired with Deteriorated Credit Quality [Member] | Performing loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	7,302
Receivables Acquired with Deteriorated Credit Quality [Member] | Performing loans [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,529
Receivables Acquired with Deteriorated Credit Quality [Member] | Performing loans [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,066
Receivables Acquired with Deteriorated Credit Quality [Member] | Performing loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	$ 74