SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) segment shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Jan. 01, 2019 USD ($)
Cash and Cash Equivalents
Reserve requirement	$ 61,200	$ 44,100
Total of the average daily required reserve	50,400	38,800
Other Investments
Other investments	14,455	42,270
Servicing Rights
Servicing fees	4,492	1,687	$ 1,708
Share-based Compensation
Company stock-based compensation cost	$ 6,200	$ 3,300	$ 2,300
Earnings Per Share
Anti-dilutive common shares excluded | shares	0	0	0
Mortgage Banking Derivatives
Fair value of derivative instruments, asset	$ 2,537	$ 2,888
Fair value of derivative instruments, liability	$ 1,276	67
Number of reportable segments | segment	5
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Stranded tax effects		(392)
Reclassification to retained earnings due to change in federal corporate tax rate	$ (392)
Fair Value Hedging
Derivative Instruments and Hedging Activity
Derivative asset	$ 102
Derivative liability		381
Buildings and leasehold improvements | Maximum
Premises and Equipment
Estimated useful lives, premises and equipment	40 years
Furniture and equipment | Maximum
Premises and Equipment
Estimated useful lives, premises and equipment	20 years
Furniture and equipment | Minimum
Premises and Equipment
Estimated useful lives, premises and equipment	3 years
Software And Computer Equipment | Maximum
Premises and Equipment
Estimated useful lives, premises and equipment	5 years
Software And Computer Equipment | Minimum
Premises and Equipment
Estimated useful lives, premises and equipment	3 years
Junior Subordinated Debt | Cash Flow Hedging
Derivative Instruments and Hedging Activity
Derivative, notional amount	$ 37,100	37,100
Core deposit premiums | Maximum
Goodwill and Intangible Assets
Acquired finite-lived intangible assets, weighted average useful life	10 years
Core deposit premiums | Minimum
Goodwill and Intangible Assets
Acquired finite-lived intangible assets, weighted average useful life	7 years
Single Family Residential Assets (SFR)
FDIC Loss-Share Receivable/Payable
Coverage period for acquisitions	10 years
Non-Single Family Assets (NSF)
FDIC Loss-Share Receivable/Payable
Coverage period for acquisitions, losses and recoveries both covered	5 years
US Premium Financing Holding Company | Insurance agent relationships
Goodwill and Intangible Assets
Acquired finite-lived intangible assets, weighted average useful life	8 years
US Premium Financing Holding Company | US Premium Finance trade name
Goodwill and Intangible Assets
Acquired finite-lived intangible assets, weighted average useful life	7 years
US Premium Financing Holding Company | Non-compete agreement
Goodwill and Intangible Assets
Acquired finite-lived intangible assets, weighted average useful life	3 years
Retained Earnings
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Reclassification to retained earnings due to change in federal corporate tax rate	$ 392	$ 0	$ 0
Accounting Standards Update 2018-02 | Accumulated Other Comprehensive Income (Loss), Net of Tax
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Reclassification to retained earnings due to change in federal corporate tax rate	(392)
Accounting Standards Update 2018-02 | Retained Earnings
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Reclassification to retained earnings due to change in federal corporate tax rate	$ 392
Visa Class B Shares
Other Investments
Shares held in other investment | shares	11,175
Other investments	$ 242
Conversation ratio	1.6298
Scenario, Forecast | Subsequent Event | Accounting Standards Update 2016-02 | Maximum
Property, Plant and Equipment [Line Items]
Operating Lease, Liability				$ 35,000
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Operating lease, right-of-use assets				35,000
Scenario, Forecast | Subsequent Event | Accounting Standards Update 2016-02 | Minimum
Property, Plant and Equipment [Line Items]
Operating Lease, Liability				30,000
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Operating lease, right-of-use assets				$ 30,000