LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Allowance for Loan Losses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	$ 78,008	$ 75,907
Provision for loan losses	0	0
Recoveries	1,465	647
Charge-offs	(1,276)	(1,189)
Allowance for loan losses, Ending balance	78,197	75,365
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			$ 1,654			$ 2,606
Allowance collectively evaluated for impairment			76,474			72,759
Allowance for purchased credit-impaired loans			69			0
Total allowance for loan losses	78,008	75,907	78,197	$ 78,008	$ 78,008	75,365
Loans individually evaluated for impairment			24,614			40,085
Loans collectively evaluated for impairment			7,108,114			4,059,640
Purchased Credit-Impaired			53,271	58,600		5,674
Total loans			7,185,999	7,314,504		4,105,399
Commercial real estate [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance		18,784
Provision for loan losses	(842)	305
Recoveries	38	14
Charge-offs	(180)	0
Allowance for loan losses, Ending balance	19,732	19,103
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			578			643
Allowance collectively evaluated for impairment			19,144			18,460
Allowance for purchased credit-impaired loans			10			0
Total allowance for loan losses	19,732	18,784	19,732		20,716	19,103
Loans individually evaluated for impairment			8,432			8,958
Loans collectively evaluated for impairment			3,086,549			1,550,691
Purchased Credit-Impaired			38,296			4,575
Total loans			3,133,277	3,093,160		1,564,224
Multifamily real estate [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance		4,562
Provision for loan losses	(1,342)	(161)
Recoveries	0	0
Charge-offs	0	0
Allowance for loan losses, Ending balance	2,853	4,401
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			68			84
Allowance collectively evaluated for impairment			2,784			4,317
Allowance for purchased credit-impaired loans			1			0
Total allowance for loan losses	2,853	4,562	2,853		4,195	4,401
Loans individually evaluated for impairment			355			1,359
Loans collectively evaluated for impairment			304,982			207,328
Purchased Credit-Impaired			1,682	1,994		0
Total loans			307,019	472,976		208,687
Construction and Land [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance		23,545
Provision for loan losses	1,716	745
Recoveries	471	108
Charge-offs	0	0
Allowance for loan losses, Ending balance	29,318	24,398
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			382			808
Allowance collectively evaluated for impairment			28,881			23,590
Allowance for purchased credit-impaired loans			55			0
Total allowance for loan losses	29,318	23,545	29,318		27,131	24,398
Loans individually evaluated for impairment			4,183			11,573
Loans collectively evaluated for impairment			624,022			419,390
Purchased Credit-Impaired			3,925			0
Total loans			632,130	574,370		430,963
Commercial business [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance		12,043
Provision for loan losses	681	778
Recoveries	720	178
Charge-offs	(139)	(107)
Allowance for loan losses, Ending balance	15,118	12,892
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			60			78
Allowance collectively evaluated for impairment			15,058			12,814
Allowance for purchased credit-impaired loans			0			0
Total allowance for loan losses	15,118	12,043	15,118		13,856	12,892
Loans individually evaluated for impairment			1,402			725
Loans collectively evaluated for impairment			1,216,477			775,854
Purchased Credit-Impaired			7,036	7,302		0
Total loans			1,224,915	1,207,944		776,579
Agricultural Business/Farmland [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance		2,821
Provision for loan losses	1,187	1,434
Recoveries	17	295
Charge-offs	(567)	(818)
Allowance for loan losses, Ending balance	4,282	3,732
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			0			3
Allowance collectively evaluated for impairment			4,282			3,729
Allowance for purchased credit-impaired loans			0			0
Total allowance for loan losses	4,282	2,821	4,282		3,645	3,732
Loans individually evaluated for impairment			563			289
Loans collectively evaluated for impairment			338,356			208,346
Purchased Credit-Impaired			1,431	1,529		0
Total loans			340,350	376,531		208,635
One- to four-family [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance		8,447
Provision for loan losses	(2,574)	(237)
Recoveries	12	6
Charge-offs	0	(75)
Allowance for loan losses, Ending balance	2,170	8,141
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			557			925
Allowance collectively evaluated for impairment			1,613			7,216
Allowance for purchased credit-impaired loans			0			0
Total allowance for loan losses	2,170	8,447	2,170		4,732	8,141
Loans individually evaluated for impairment			9,277			16,036
Loans collectively evaluated for impairment			901,156			525,876
Purchased Credit-Impaired			286	1,066		1,092
Total loans			910,719	952,633		543,004
Consumer [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance		483
Provision for loan losses	2,822	245
Recoveries	207	46
Charge-offs	(390)	(189)
Allowance for loan losses, Ending balance	3,541	585
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			9			65
Allowance collectively evaluated for impairment			3,529			520
Allowance for purchased credit-impaired loans			3			0
Total allowance for loan losses	3,541	483	3,541		902	585
Loans individually evaluated for impairment			402			1,145
Loans collectively evaluated for impairment			636,572			372,155
Purchased Credit-Impaired			615			7
Total loans			637,589	$ 636,890		373,307
Unallocated Financing Receivables [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance		5,222
Provision for loan losses	(1,648)	(3,109)
Recoveries	0	0
Charge-offs	0	0
Allowance for loan losses, Ending balance	1,183	2,113
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			0			0
Allowance collectively evaluated for impairment			1,183			2,113
Allowance for purchased credit-impaired loans			0			0
Total allowance for loan losses	$ 1,183	$ 5,222	1,183		$ 2,831	2,113
Loans individually evaluated for impairment			0			0
Loans collectively evaluated for impairment			0			0
Purchased Credit-Impaired			0			0
Total loans			$ 0			$ 0