Consolidated statements of cash flows - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Cash flows from operating activities:
Net (loss) income	$ (5,599)	$ 23,966	$ 18,019	$ 62,242
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization of fixed assets			4,924	3,830
Amortization of core deposit and other intangibles			3,825	3,180
Capitalization of mortgage servicing rights			(33,415)	(30,319)
Net change in fair value of mortgage servicing rights	7,175	10,611	42,251	23,832
Stock-based compensation expense			7,253	5,621
Provision for credit losses	45,834	1,831	97,837	4,103
Provision for credit losses on unfunded commitments	9,567	0	13,050	0
Provision for mortgage loan repurchases	901	107	2,128	255
Accretion of yield on purchased loans			(3,080)	(6,030)
Accretion of discounts and amortization of premiums on securities, net			4,431	2,113
Gain from securities, net	(583)	20	(618)	(75)
Originations of loans held for sale			(4,739,497)	(3,477,658)
Repurchases of loans held for sale			0	(9,919)
Proceeds from sale of loans held for sale			4,690,135	3,527,632
Gain on sale and change in fair value of loans held for sale			(202,336)	(72,749)
Net loss (gain) or write-downs of other real estate owned	1,505	126	1,368	(112)
Loss (gain) on other assets	(226)	(44)	156	(52)
Relief of goodwill			0	100
Provision for deferred income taxes	(15,163)	(1,449)	(31,543)	(5,900)
Changes in:
Operating leases			1,309	0
Other assets and interest receivable			(77,566)	(50,509)
Accrued expenses and other liabilities			59,325	4,243
Net cash used in operating activities			(142,044)	(16,172)
Activity in available-for-sale securities:
Sales	28,257	22,740	28,257	24,498
Maturities, prepayments and calls	67,886	28,694	140,246	78,861
Purchases			(214,285)	(92,059)
Net change in loans			(133,893)	(295,791)
Purchases of FHLB stock			(515)	(2,544)
Proceeds from sale of mortgage servicing rights			0	29,160
Purchases of premises and equipment			(2,652)	(4,052)
Proceeds from the sale of premises and equipment			0	1,275
Proceeds from the sale of other real estate owned	1,411	854	5,561	2,718
Net cash paid in business combination			248,439	171,032
Net cash provided by (used in) investing activities			71,158	(86,902)
Cash flows from financing activities:
Net increase in demand deposits			1,043,928	231,763
Net decrease in time deposits			(201,659)	(70,594)
Net increase in securities sold under agreements to repurchase			5,532	1,546
Net (decrease) increase in FHLB advances			(50,000)	68,235
Issuance of subordinated debt			100,000	0
Payments for issuance costs associated with subordinated debt			(1,890)	0
Proceeds from other borrowings			15,000	0
Share based compensation withholding payments			(1,268)	(3,552)
Net proceeds from sale of common stock under employee stock purchase program			978	804
Dividends paid			(10,025)	(7,487)
Net cash provided by financing activities			900,596	220,715
Net change in cash and cash equivalents			829,710	117,641
Cash and cash equivalents at beginning of the period			232,681	125,356	$ 125,356
Cash and cash equivalents at end of the period	1,062,391	242,997	1,062,391	242,997	232,681
Supplemental cash flow information:
Interest paid			36,498	41,463
Taxes paid			16,449	21,377
Supplemental noncash disclosures:
Transfers from loans to other real estate owned	573	1,535	1,579	3,565
Transfers from other real estate owned to premises and equipment			841	0
Transfers from premises and equipment to other real estate owned			0	2,640
Loans provided for sales of other real estate owned	0	0	0	166
Transfers from loans to loans held for sale			9,304	5,460
Transfers from loans held for sale to loans			(49,508)	11,476
Stock consideration paid in business combination			480,867	0
Trade date payable - securities			1,214	3,671	$ 0
Trade date receivable - securities			0	0
Dividends declared not paid on restricted stock units	$ 139	$ 168	139	168
Decrease to retained earnings for adoption of new accounting standards			25,018	1,309
Right-of-use assets obtained in exchange for operating lease liabilities			$ 806	$ 39,011