SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) segment shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares
Cash and Cash Equivalents
Total of the average daily required reserve	$ 38,800	$ 37,800
Servicing Rights
Servicing fees	$ 1,687	1,708	$ 1,268
FDIC Loss-Share Receivable/Payable
Clawback discount period	10 years
Share-based Compensation
Company stock-based compensation cost	$ 3,300	$ 2,300	$ 1,500
Earnings Per Share
Anti-dilutive common shares excluded | shares	0	0	0
Mortgage Banking Derivatives
Fair value of derivative instruments, asset	$ 2,888	$ 4,314
Fair value of derivative instruments, liability	$ 67	0
Number of reportable segments | segment	5
Fair Value Hedging
Derivative Instruments and Hedging Activity
Derivative fair value liability related to cash flow hedge	$ 381	978
Buildings | 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 fair value liability related to cash flow hedge	$ 37,100	$ 37,100
Core Deposits | Maximum
Goodwill and Intangible Assets
Finite-lived intangible asset, useful life	10 years
Core Deposits | Minimum
Goodwill and Intangible Assets
Finite-lived intangible asset, 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	8 years
Coverage period for acquisitions, losses and recoveries both covered	5 years