Fair Value - Estimated fair value and related carrying amount, of the Company's financial instruments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Financial assets:
Loans held for sale	$ 456,141	$ 87,393	$ 59,363
Interest Receivable	97,800
Financial liabilities:
Increase (Decrease) in Loans Held-for-sale	737,130	280,684
Other borrowings	700,097	700,105	700,103
Other derivative financial instruments (Mortgage banking related)	979,155	43,287	30,485
Carrying Amount
Financial assets:
Cash and cash equivalents	4,471,639	719,194	688,704
Investment securities	3,747,128	1,862,258	2,005,171
Loans held for sale	456,141		59,363
Loans held for sale		87,393
Loans, net of allowance for loan losses	24,797,656	11,229,892	11,313,113
Accrued interest receivable	109,954	36,131	36,774
Mortgage servicing rights	34,578	28,674	30,525
Interest rate swap - non-designated hedge	931,386	21,700	15,350
Other derivative financial instruments (mortgage banking related)	21,650	2,702	902
Financial liabilities:
Deposits	29,969,954	12,023,787	12,177,096
Federal Funds Purchased and Securities Sold under Agreements to Repurchase.	706,723		298,741
Federal funds purchased and securities sold under agreements to repurchase		269,072
Other borrowings	1,089,637		815,936
Other borrowings		815,771
Accrued interest payable	11,464	5,036	4,916
Interest rate swap - non-designated hedge	933,357	23,723	16,693
Interest rate swap - cash flow hedge	41,798	19,564	13,791
Other derivative financial instruments (Mortgage banking related)		86	789
Fair Value
Financial assets:
Cash and cash equivalents	4,471,639	719,194	688,704
Investment securities	3,747,128	1,862,258	2,005,171
Loans held for sale	456,141		59,363
Loans held for sale		87,393
Loans, net of allowance for loan losses	25,140,453	11,259,351	11,452,003
Accrued interest receivable	109,954	36,131	36,774
Mortgage servicing rights	34,578	28,674	30,525
Interest rate swap - non-designated hedge	931,386	21,700	15,350
Other derivative financial instruments (mortgage banking related)	21,650	2,702	902
Financial liabilities:
Deposits	29,989,297	11,302,479	11,406,477
Federal Funds Purchased and Securities Sold under Agreements to Repurchase.	706,723		298,741
Federal funds purchased and securities sold under agreements to repurchase		269,072
Other borrowings	1,087,636		818,210
Other borrowings		818,294
Accrued interest payable	11,464	5,036	4,916
Interest rate swap - non-designated hedge	933,357	23,723	16,693
Interest rate swap - cash flow hedge	41,798	19,564	13,791
Other derivative financial instruments (Mortgage banking related)		86	789
Commitments to extend credit		7,622
Commitments to extend credit | Fair Value
Financial liabilities:
Commitments to extend credit	82,940		36,031
Quoted Prices In Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	4,471,639	719,194	688,704
Investment securities	185,199	49,124	49,124
Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities	3,561,929	1,813,134	1,956,047
Loans held for sale	456,141		59,363
Loans held for sale		87,393
Accrued interest receivable	11,086	7,528	8,500
Interest rate swap - non-designated hedge	931,386	21,700	15,350
Other derivative financial instruments (mortgage banking related)	21,650	2,702	902
Financial liabilities:
Deposits	29,989,297	11,302,479	11,406,477
Federal Funds Purchased and Securities Sold under Agreements to Repurchase.	706,723		298,741
Federal funds purchased and securities sold under agreements to repurchase		269,072
Other borrowings	1,087,636		818,210
Other borrowings		818,294
Accrued interest payable	11,464	5,036	4,916
Interest rate swap - non-designated hedge	933,357	23,723	16,693
Interest rate swap - cash flow hedge	41,798	19,564	13,791
Other derivative financial instruments (Mortgage banking related)		86	789
Commitments to extend credit		7,622
Significant Other Observable Inputs (Level 2) | Commitments to extend credit
Financial liabilities:
Commitments to extend credit	82,940		36,031
Significant Unobservable Inputs (Level 3)
Financial assets:
Loans, net of allowance for loan losses	25,140,453	11,259,351	11,452,003
Accrued interest receivable	98,868	28,603	28,274
Mortgage servicing rights	$ 34,578	$ 28,674	$ 30,525