LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Allowance for Loan Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Mar. 31, 2016
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			$ 85,997	$ 78,008
Provision for loan losses	$ 2,000	$ 2,000	6,000	4,000
Recoveries	776	1,557	3,153	4,761
Charge-offs	(2,262)	(655)	(6,050)	(2,549)
Allowance for loan losses, Ending balance	89,100	84,220	89,100	84,220
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					$ 773			$ 1,809
Allowance collectively evaluated for impairment					88,320			82,378
Allowance for purchased credit-impaired loans					7			33
Total allowance for loan losses	89,100	84,220	85,997	78,008	89,100	$ 88,586	$ 85,997	84,220	$ 81,318
Loans individually evaluated for impairment					33,531			34,487
Loans collectively evaluated for impairment					7,717,697			7,325,476
Purchased Credit-Impaired					23,221		32,322	38,674
Total loans					7,774,449		7,451,148	7,398,637
Commercial real estate [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			20,993	20,716
Provision for loan losses	(236)	(337)	2,716	(788)
Recoveries	19	34	353	98
Charge-offs	(584)	0	(631)	(180)
Allowance for loan losses, Ending balance	23,431	19,846	23,431	19,846
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					263			832
Allowance collectively evaluated for impairment					23,168			19,014
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	23,431	19,846	20,993	20,716	23,431	24,232	20,993	19,846	20,149
Loans individually evaluated for impairment					13,866			16,630
Loans collectively evaluated for impairment					3,332,724			3,214,042
Purchased Credit-Impaired					15,684			28,544
Total loans					3,362,274			3,259,216
Multifamily real estate [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			1,360	4,195
Provision for loan losses	63	(79)	254	(2,759)
Recoveries	0	0	11	0
Charge-offs	0	0	0	0
Allowance for loan losses, Ending balance	1,625	1,436	1,625	1,436
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					0			65
Allowance collectively evaluated for impairment					1,625			1,371
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	1,625	1,436	1,360	4,195	1,625	1,562	1,360	1,436	1,515
Loans individually evaluated for impairment					0			351
Loans collectively evaluated for impairment					311,533			266,274
Purchased Credit-Impaired					173		139	258
Total loans					311,706		248,150	266,883
Construction and Land [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			34,252	27,131
Provision for loan losses	2,037	1,269	(6,010)	5,404
Recoveries	73	673	1,180	1,268
Charge-offs	0	0	0	0
Allowance for loan losses, Ending balance	29,422	33,803	29,422	33,803
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					67			396
Allowance collectively evaluated for impairment					29,348			33,374
Allowance for purchased credit-impaired loans					7			33
Total allowance for loan losses	29,422	33,803	34,252	27,131	29,422	27,312	34,252	33,803	31,861
Loans individually evaluated for impairment					1,871			4,137
Loans collectively evaluated for impairment					872,783			789,037
Purchased Credit-Impaired					3,759			4,153
Total loans					878,413			797,327
Commercial business [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			16,533	13,856
Provision for loan losses	(555)	(1,351)	4,489	1,519
Recoveries	577	433	921	1,775
Charge-offs	(491)	(333)	(3,286)	(643)
Allowance for loan losses, Ending balance	18,657	16,507	18,657	16,507
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					52			54
Allowance collectively evaluated for impairment					18,605			16,453
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	18,657	16,507	16,533	13,856	18,657	19,126	16,533	16,507	17,758
Loans individually evaluated for impairment					5,899			2,026
Loans collectively evaluated for impairment					1,302,902			1,181,558
Purchased Credit-Impaired					2,608		3,821	4,264
Total loans					1,311,409		1,207,879	1,187,848
Agricultural Business/Farmland [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			2,967	3,645
Provision for loan losses	1,141	80	2,113	(284)
Recoveries	1	(138)	133	39
Charge-offs	(1,001)	0	(1,264)	(567)
Allowance for loan losses, Ending balance	3,949	2,833	3,949	2,833
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					196			0
Allowance collectively evaluated for impairment					3,753			2,833
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	3,949	2,833	2,967	3,645	3,949	3,808	2,967	2,833	2,891
Loans individually evaluated for impairment					6,495			2,758
Loans collectively evaluated for impairment					332,754			379,710
Purchased Credit-Impaired					683		684	807
Total loans					339,932		369,156	383,275
One- to four-family [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			2,238	4,732
Provision for loan losses	22	(404)	(460)	(3,468)
Recoveries	8	482	262	1,052
Charge-offs	0	(92)	0	(126)
Allowance for loan losses, Ending balance	2,040	2,190	2,040	2,190
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					184			456
Allowance collectively evaluated for impairment					1,856			1,734
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	2,040	2,190	2,238	4,732	2,040	2,010	2,238	2,190	2,204
Loans individually evaluated for impairment					5,182			8,270
Loans collectively evaluated for impairment					864,109			838,328
Purchased Credit-Impaired					265		274	301
Total loans					869,556		813,077	846,899
Consumer [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			4,104	902
Provision for loan losses	117	348	488	3,536
Recoveries	98	73	293	529
Charge-offs	(186)	(230)	(869)	(1,033)
Allowance for loan losses, Ending balance	4,016	3,934	4,016	3,934
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					11			6
Allowance collectively evaluated for impairment					4,005			3,928
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	4,016	3,934	4,104	902	4,016	3,987	4,104	3,934	3,743
Loans individually evaluated for impairment					218			315
Loans collectively evaluated for impairment					700,892			656,527
Purchased Credit-Impaired					49			347
Total loans					701,159			657,189
Unallocated Financing Receivables [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			3,550	2,831
Provision for loan losses	(589)	2,474	2,410	840
Recoveries	0	0	0	0
Charge-offs	0	0	0	0
Allowance for loan losses, Ending balance	5,960	3,671	5,960	3,671
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					0			0
Allowance collectively evaluated for impairment					5,960			3,671
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	$ 5,960	$ 3,671	$ 3,550	$ 2,831	5,960	$ 6,549	$ 3,550	3,671	$ 1,197
Loans individually evaluated for impairment					0			0
Loans collectively evaluated for impairment					0			0
Purchased Credit-Impaired					0			0
Total loans					$ 0			$ 0