SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands		12 Months Ended
	Jan. 01, 2020 USD ($)	Dec. 31, 2020 USD ($) segment rating shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Cash and Cash Equivalents
Cash reserve deposit required rate		0.00%
Reserve requirement		$ 0	$ 109,700
Other Investments
Other investments		28,202	66,919
Servicing Rights
Servicing fees		$ 37,200	19,900	$ 4,500
Allowance for Credit Losses
Number of risk ratings | rating		9
Number of risk ratings, pass | rating		5
Number of risk ratings, criticized | rating		4
Leases
Initial lease terms		13 years
Share-based Compensation
Company stock-based compensation cost		$ 3,900	$ 3,400	$ 6,200
Earnings Per Share
Anti-dilutive common shares excluded (in shares) | shares		197,765	0	0
Derivative Instruments and Hedging Activity
Derivative, fixed interest rate			4.11%
Mortgage Banking Derivatives
Fair value of derivative instruments, asset		$ 51,800	$ 7,800
Fair value of derivative instruments, liability		$ 16,400	4,500
Number of reportable segments | segment		5
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Financing receivable allowance for unfunded commitments	$ 12,700
Retained earnings (accumulated deficit)		$ 671,510	507,950
Deferred tax assets, net		33,314	2,180
Deferred interest on financing receivable allowance for credit losses		718
Allowance for loan losses		$ 199,422	38,189	$ 28,819	$ 25,791
Minimum | Core deposit premiums
Goodwill and Intangible Assets
Acquired finite-lived intangible assets, weighted average useful life		7 years
Maximum | Core deposit premiums
Goodwill and Intangible Assets
Acquired finite-lived intangible assets, weighted average useful life		10 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
Financing receivable
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Interest receivable		$ 73,400
Available-for-sale Securities
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Interest receivable		$ 3,600
Accounting Standards Update 2016-13
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Retained earnings (accumulated deficit)	56,700
Deferred tax assets, net	19,000
Addition due to acquisition	15,600
Cash Flow Hedging
Derivative Instruments and Hedging Activity
Derivative liability			187
Buildings and leasehold improvements | Maximum
Premises and Equipment
Estimated useful lives, premises and equipment		40 years
Furniture and equipment | Minimum
Premises and Equipment
Estimated useful lives, premises and equipment		3 years
Furniture and equipment | Maximum
Premises and Equipment
Estimated useful lives, premises and equipment		20 years
Software And Computer 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
Junior Subordinated Debt | Cash Flow Hedging
Derivative Instruments and Hedging Activity
Derivative, notional amount			$ 37,100
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	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
Accounting Standards Update 2016-13
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Allowance for loan losses	$ 78,700		$ 78,661
Visa Class B Shares
Other Investments
Shares held in other investment (in shares) | shares		57,611
Other investments		$ 242
Conversation ratio		1.6228