Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Financial assets:
Securities Purchased under Agreements to Resell	$ 102.0	$ 0.0
Investment debt securities available-for-sale	6,283.8	4,820.5
Investment debt securities held-to-maturity	3,029.1	1,667.5
Derivative assets	26.9	28.1
Financial liabilities:
Derivative liabilities	62.7	18.2
Carrying Amount
Financial assets:
Cash and cash equivalents	3,811.3	2,344.8
Securities Purchased under Agreements to Resell	102.0
Investment debt securities available-for-sale	6,283.8	4,820.5
Investment debt securities held-to-maturity	3,218.7	1,687.6
Accrued interest receivable	81.1	47.4
Mortgage servicing rights, net	32.7	28.2
Loans held for sale	178.1	30.1
Net loans held for investment	16,697.8	9,209.4
Derivative assets	26.9	28.1
Deferred compensation plan assets	19.7	21.4
Total financial assets	30,452.1	18,217.5
Financial liabilities:
Total deposits, excluding time deposits	26,489.9	15,303.1
Time deposits	1,598.4	966.5
Securities sold under repurchase agreements	1,071.0	1,051.1
Accrued interest payable	6.0	3.7
Long-term debt	120.4	112.4
Subordinated debentures held by subsidiary trusts	163.1	87.0
Derivative liabilities	62.7	18.2
Deferred compensation plan liabilities	19.7	21.4
Total financial liabilities	29,531.2	17,563.4
Estimated Fair Value
Financial assets:
Cash and cash equivalents	3,811.3	2,344.8
Securities Purchased under Agreements to Resell	102.0
Investment debt securities available-for-sale	6,283.8	4,820.5
Investment debt securities held-to-maturity	3,029.1	1,667.5
Accrued interest receivable	81.1	47.4
Mortgage servicing rights, net	35.8	28.2
Loans held for sale	178.1	30.1
Net loans held for investment	16,676.6	9,254.3
Derivative assets	26.9	28.1
Deferred compensation plan assets	19.7	21.4
Total financial assets	30,244.4	18,242.3
Financial liabilities:
Total deposits, excluding time deposits	26,489.9	15,303.1
Time deposits	1,579.8	963.1
Securities sold under repurchase agreements	1,071.0	1,051.1
Accrued interest payable	6.0	3.7
Long-term debt	122.8	120.7
Subordinated debentures held by subsidiary trusts	162.3	85.5
Derivative liabilities	62.7	18.2
Deferred compensation plan liabilities	19.7	21.4
Total financial liabilities	29,514.2	17,566.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	3,811.3	2,344.8
Securities Purchased under Agreements to Resell	0.0
Investment debt securities available-for-sale	644.9	684.7
Investment debt securities held-to-maturity	296.2	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	0.0	0.0
Loans held for sale	0.0	0.0
Net loans held for investment	0.0	0.0
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	4,752.4	3,029.5
Financial liabilities:
Total deposits, excluding time deposits	26,489.9	15,303.1
Time deposits	0.0	0.0
Securities sold under repurchase agreements	0.0	0.0
Accrued interest payable	0.0	0.0
Long-term debt	0.0	0.0
Subordinated debentures held by subsidiary trusts	0.0	0.0
Derivative liabilities	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Total financial liabilities	26,489.9	15,303.1
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Securities Purchased under Agreements to Resell	102.0
Investment debt securities available-for-sale	5,638.9	4,135.8
Investment debt securities held-to-maturity	2,732.9	1,667.5
Accrued interest receivable	81.1	47.4
Mortgage servicing rights, net	35.8	28.2
Loans held for sale	178.1	30.1
Net loans held for investment	16,575.5	9,242.6
Derivative assets	26.9	28.1
Deferred compensation plan assets	19.7	21.4
Total financial assets	25,390.9	15,201.1
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,579.8	963.1
Securities sold under repurchase agreements	1,071.0	1,051.1
Accrued interest payable	6.0	3.7
Long-term debt	122.8	120.7
Subordinated debentures held by subsidiary trusts	162.3	85.5
Derivative liabilities	62.7	18.2
Deferred compensation plan liabilities	19.7	21.4
Total financial liabilities	3,024.3	2,263.7
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Securities Purchased under Agreements to Resell	0.0
Investment debt securities available-for-sale	0.0	0.0
Investment debt securities held-to-maturity	0.0	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	0.0	0.0
Loans held for sale	0.0	0.0
Net loans held for investment	101.1	11.7
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	101.1	11.7
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	0.0	0.0
Securities sold under repurchase agreements	0.0	0.0
Accrued interest payable	0.0	0.0
Long-term debt	0.0	0.0
Subordinated debentures held by subsidiary trusts	0.0	0.0
Derivative liabilities	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Total financial liabilities	$ 0.0	$ 0.0