NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017 USD ($) Office $ / shares		Sep. 30, 2017 USD ($) $ / shares		Jun. 30, 2017 USD ($) $ / shares		Mar. 31, 2017 USD ($) $ / shares		Dec. 31, 2016 USD ($) $ / shares		Sep. 30, 2016 USD ($) $ / shares		Jun. 30, 2016 USD ($) $ / shares		Mar. 31, 2016 USD ($) $ / shares		Dec. 31, 2018	Dec. 31, 2017 USD ($) Office Segment Customer $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Nature of Operations [Abstract]
Subordinated notes payable | $	$ 113,612								$ 0									$ 113,612	$ 0
Number of retail banking offices | Office	28																	28
Loans [Abstract]
Number of months for which borrower is required to demonstrate performance in accordance with the loan terms and conditions prior to returning a loan to accrual status																		6 months
Accounting for Goodwill and Other Intangible Assets [Abstract]
Goodwill | $	$ 55,638								55,638									$ 55,638	55,638
Numerator [Abstract]
Net Income per the Consolidated Statements of Operations | $	$ 15,423		$ 13,313		$ 11,989		$ 11,157		$ 732		$ 10,537		$ 11,208		$ 50,037			51,882	72,514	$ 44,772
Less: Dividends paid on earnings allocated to participating securities | $																		(131)	(109)	(136)
Income attributable to Common Stock | $																		$ 51,751	$ 72,405	$ 44,636
Weighted average common shares outstanding, including participating securities (in shares)																		37,593,715	36,898,951	36,477,854
Less: weighted average participating securities (in shares)																		(163,056)	(186,058)	(245,037)
Weighted average common shares outstanding (in shares)																		37,430,659	36,712,893	36,232,817
Basic EPS (in dollars per share) | $ / shares	$ 0.41	[1]	$ 0.36	[1]	$ 0.32	[1]	$ 0.30	[1]	$ 0.02	[1]	$ 0.29	[1]	$ 0.30	[1]	$ 1.37	[1]		$ 1.38	$ 1.97	$ 1.24
Income attributable to Common Stock | $																		$ 51,751	$ 72,405	$ 44,636
Weighted average common shares outstanding (in shares)																		37,430,659	36,712,893	36,232,817
Weighted average common equivalent shares outstanding (in shares)																		79,790	51,193	89,516
Weighted average common and equivalent shares outstanding (in shares)																		37,510,449	36,764,086	36,322,333
Diluted EPS (in dollars per share) | $ / shares	$ 0.41	[1]	$ 0.35	[1]	$ 0.32	[1]	$ 0.30	[1]	$ 0.02	[1]	$ 0.29	[1]	$ 0.30	[1]	$ 1.36	[1]		$ 1.38	$ 1.97	$ 1.23
Disclosure About Segments of an Enterprise and Related Information [Abstract]
Number of reportable segments | Segment																		1
Number of operating segments | Segment																		1
Number of customers that account for more than 10% of consolidated revenue | Customer																		0
Recently Issued Accounting Standards [Abstract]
Corporate income tax rate																		35.00%
Plan [Member]
Recently Issued Accounting Standards [Abstract]
Corporate income tax rate																	21.00%
Buildings [Member] | Minimum [Member]
Premises and Fixed Assets, Net [Abstract]
Percentage of Depreciation																		2.22%
Buildings [Member] | Maximum [Member]
Premises and Fixed Assets, Net [Abstract]
Percentage of Depreciation																		2.50%
Furniture, Fixtures and Equipment [Member]
Premises and Fixed Assets, Net [Abstract]
Percentage of Depreciation																		10.00%
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average shares excluded from earnings per share calculation (in shares)																		0	77,432	126,172

[1] The quarterly EPS amounts, when added, may not coincide with the full fiscal year EPS reported on the Consolidated Statements of Operations due to differences in the computed weighted average shares outstanding as well as rounding differences.