SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) segment shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Jan. 01, 2020 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2016 USD ($)
Cash and Cash Equivalents
Reserve requirement	$ 109,700	$ 61,200
Other Investments
Other investments	66,919	14,455
Servicing Rights
Servicing fees	$ 19,900	4,500	$ 1,700
Leases
Initial lease terms	10 years
Share-based Compensation
Company stock-based compensation cost	$ 3,400	$ 6,200	$ 3,300
Earnings Per Share
Anti-dilutive common shares excluded (in shares) | shares	0	0	0
Mortgage Banking Derivatives
Fair value of derivative instruments, asset	$ 7,800	$ 2,500
Fair value of derivative instruments, liability	$ 4,500	1,300
Number of reportable segments | segment	5
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Operating lease, right-of-use assets	$ 39,321
Operating lease liabilities	42,262
Cumulative effect of change in accounting		(276)	$ 28
Allowance for loan losses	38,189	28,819	25,791			$ 23,920
Fair Value Hedging
Derivative Instruments and Hedging Activity
Derivative liability	$ 187
Derivative asset		102
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	10 years
Core deposit premiums | Minimum
Goodwill and Intangible Assets
Acquired finite-lived intangible assets, weighted average useful life	7 years	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	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	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	3 years
Retained Earnings
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Cumulative effect of change in accounting		$ (276)	$ 28
Accounting Standards Update 2016-02
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Operating lease, right-of-use assets					$ 27,300
Operating lease liabilities					29,700
Cumulative effect of change in accounting					$ 276
Accounting Standards Update 2016-13 | Maximum
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Allowance for loan losses				$ 120,000
Unfunded commitments				20,000
Accounting Standards Update 2016-13 | Minimum
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Allowance for loan losses				85,000
Unfunded commitments				$ 10,000
Visa Class B Shares
Other Investments
Shares held in other investment (in shares) | shares	57,611
Other investments	$ 242
Conversation ratio	1.6228