LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Allowance for Loan Losses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Dec. 31, 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	0
Recoveries	678	1,465
Charge-offs	(2,148)	(1,276)
Allowance for loan losses, Ending balance	86,527	78,197
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			$ 1,344		$ 1,654
Allowance collectively evaluated for impairment			85,164		76,474
Allowance for purchased credit-impaired loans			19		69
Total allowance for loan losses	85,997	78,008	86,527	$ 85,997	78,197
Loans individually evaluated for impairment			27,257		24,614
Loans collectively evaluated for impairment			7,363,497		7,108,114
Purchased Credit-Impaired			30,501	32,322	53,271
Total loans			7,421,255	7,451,148	7,185,999
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	(591)	(842)
Recoveries	70	38
Charge-offs	0	(180)
Allowance for loan losses, Ending balance	20,472	19,732
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			419		578
Allowance collectively evaluated for impairment			20,053		19,144
Allowance for purchased credit-impaired loans			0		10
Total allowance for loan losses	20,993	20,716	20,472	20,993	19,732
Loans individually evaluated for impairment			9,506		8,432
Loans collectively evaluated for impairment			3,341,177		3,086,549
Purchased Credit-Impaired			22,030		38,296
Total loans			3,372,713		3,133,277
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	18	(1,342)
Recoveries	0	0
Charge-offs	0	0
Allowance for loan losses, Ending balance	1,378	2,853
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			62		68
Allowance collectively evaluated for impairment			1,316		2,784
Allowance for purchased credit-impaired loans			0		1
Total allowance for loan losses	1,360	4,195	1,378	1,360	2,853
Loans individually evaluated for impairment			347		355
Loans collectively evaluated for impairment			253,725		304,982
Purchased Credit-Impaired			174	139	1,682
Total loans			254,246	248,150	307,019
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	(4,871)	1,716
Recoveries	83	471
Charge-offs	0	0
Allowance for loan losses, Ending balance	29,464	29,318
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			323		382
Allowance collectively evaluated for impairment			29,122		28,881
Allowance for purchased credit-impaired loans			19		55
Total allowance for loan losses	34,252	27,131	29,464	34,252	29,318
Loans individually evaluated for impairment			4,006		4,183
Loans collectively evaluated for impairment			795,887		624,022
Purchased Credit-Impaired			3,777		3,925
Total loans			803,670		632,130
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	4,688	681
Recoveries	173	720
Charge-offs	(1,626)	(139)
Allowance for loan losses, Ending balance	19,768	15,118
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			62		60
Allowance collectively evaluated for impairment			19,706		15,058
Allowance for purchased credit-impaired loans			0		0
Total allowance for loan losses	16,533	13,856	19,768	16,533	15,118
Loans individually evaluated for impairment			2,710		1,402
Loans collectively evaluated for impairment			1,218,415		1,216,477
Purchased Credit-Impaired			3,416	3,821	7,036
Total loans			1,224,541	1,207,879	1,224,915
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	324	1,187
Recoveries	113	17
Charge-offs	(159)	(567)
Allowance for loan losses, Ending balance	3,245	4,282
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			238		0
Allowance collectively evaluated for impairment			3,007		4,282
Allowance for purchased credit-impaired loans			0		0
Total allowance for loan losses	2,967	3,645	3,245	2,967	4,282
Loans individually evaluated for impairment			3,815		563
Loans collectively evaluated for impairment			308,834		338,356
Purchased Credit-Impaired			725	684	1,431
Total loans			313,374	369,156	340,350
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	(409)	(2,574)
Recoveries	145	12
Charge-offs	0	0
Allowance for loan losses, Ending balance	1,974	2,170
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			229		557
Allowance collectively evaluated for impairment			1,745		1,613
Allowance for purchased credit-impaired loans			0		0
Total allowance for loan losses	2,238	4,732	1,974	2,238	2,170
Loans individually evaluated for impairment			6,653		9,277
Loans collectively evaluated for impairment			796,026		901,156
Purchased Credit-Impaired			312	274	286
Total loans			802,991	813,077	910,719
Consumer [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	4,104	902
Provision for loan losses	5	2,822
Recoveries	94	207
Charge-offs	(363)	(390)
Allowance for loan losses, Ending balance	3,840	3,541
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			11		9
Allowance collectively evaluated for impairment			3,829		3,529
Allowance for purchased credit-impaired loans			0		3
Total allowance for loan losses	4,104	902	3,840	4,104	3,541
Loans individually evaluated for impairment			220		402
Loans collectively evaluated for impairment			649,433		636,572
Purchased Credit-Impaired			67		615
Total loans			649,720		637,589
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	2,836	(1,648)
Recoveries	0	0
Charge-offs	0	0
Allowance for loan losses, Ending balance	6,386	1,183
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment			0		0
Allowance collectively evaluated for impairment			6,386		1,183
Allowance for purchased credit-impaired loans			0		0
Total allowance for loan losses	$ 3,550	$ 2,831	6,386	$ 3,550	1,183
Loans individually evaluated for impairment			0		0
Loans collectively evaluated for impairment			0		0
Purchased Credit-Impaired			0		0
Total loans			$ 0		$ 0