Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Financial assets:
Estimated Fair Value	$ 6,946.1	$ 4,820.5
Held-to-maturity investment securities, estimated fair value	3,052.2	1,667.5
Derivative assets	45.2	28.1
Deferred compensation plan assets	18.7	21.4
Financial liabilities:
Derivative liabilities	159.6	18.2
Deferred compensation plan liabilities	18.7	21.4
Carrying Amount
Financial assets:
Cash and cash equivalents	870.5	2,344.8
Estimated Fair Value	6,946.1	4,820.5
Held-to-maturity investment securities, estimated fair value	3,451.8	1,687.6
Accrued interest receivable	118.3	47.4
Mortgage servicing rights, net	31.1	28.2
Loans Held-for-sale, Fair Value Disclosure	79.9	30.1
Net loans held for investment	17,879.1	9,209.4
Derivative assets	45.2	28.1
Deferred compensation plan assets	18.7	21.4
Total financial assets	29,440.7	18,217.5
Financial liabilities:
Total deposits, excluding time deposits	23,145.2	15,303.1
Time deposits	1,928.4	966.5
Securities sold under repurchase agreements	1,052.9	1,051.1
Federal Home Loan Bank Borrowings, Fair Value Disclosure	2,327.0
Accrued interest payable	14.5	3.7
Long-term debt	120.8	112.4
Subordinated debentures held by subsidiary trusts	163.1	87.0
Derivative liabilities	159.6	18.2
Deferred compensation plan liabilities	18.7	21.4
Total financial liabilities	28,930.2	17,563.4
Estimated Fair Value
Financial assets:
Cash and cash equivalents	870.5	2,344.8
Estimated Fair Value	6,946.1	4,820.5
Held-to-maturity investment securities, estimated fair value	3,052.2	1,667.5
Accrued interest receivable	118.3	47.4
Mortgage servicing rights, net	37.4	28.2
Loans Held-for-sale, Fair Value Disclosure	79.9	30.1
Net loans held for investment	17,552.1	9,254.3
Derivative assets	45.2	28.1
Deferred compensation plan assets	18.7	21.4
Total financial assets	28,720.4	18,242.3
Financial liabilities:
Total deposits, excluding time deposits	23,145.2	15,303.1
Time deposits	1,876.1	963.1
Securities sold under repurchase agreements	1,052.9	1,051.1
Federal Home Loan Bank Borrowings, Fair Value Disclosure	2,327.0
Accrued interest payable	14.5	3.7
Long-term debt	116.3	120.7
Subordinated debentures held by subsidiary trusts	155.8	85.5
Derivative liabilities	159.6	18.2
Deferred compensation plan liabilities	18.7	21.4
Total financial liabilities	28,866.1	17,566.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	870.5	2,344.8
Estimated Fair Value	0.0	684.7
Held-to-maturity investment securities, estimated fair value	0.0	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	0.0	0.0
Loans Held-for-sale, Fair Value Disclosure	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	870.5	3,029.5
Financial liabilities:
Total deposits, excluding time deposits	23,145.2	15,303.1
Time deposits	0.0	0.0
Securities sold under repurchase agreements	0.0	0.0
Federal Home Loan Bank Borrowings, Fair Value Disclosure	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	23,145.2	15,303.1
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Estimated Fair Value	6,946.1	4,135.8
Held-to-maturity investment securities, estimated fair value	3,052.2	1,667.5
Accrued interest receivable	118.3	47.4
Mortgage servicing rights, net	37.4	28.2
Loans Held-for-sale, Fair Value Disclosure	79.9	30.1
Net loans held for investment	17,513.0	9,242.6
Derivative assets	45.2	28.1
Deferred compensation plan assets	18.7	21.4
Total financial assets	27,810.8	15,201.1
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,876.1	963.1
Securities sold under repurchase agreements	1,052.9	1,051.1
Federal Home Loan Bank Borrowings, Fair Value Disclosure	2,327.0
Accrued interest payable	14.5	3.7
Long-term debt	116.3	120.7
Subordinated debentures held by subsidiary trusts	155.8	85.5
Derivative liabilities	159.6	18.2
Deferred compensation plan liabilities	18.7	21.4
Total financial liabilities	5,720.9	2,263.7
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Estimated Fair Value	0.0	0.0
Held-to-maturity investment securities, estimated fair value	0.0	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	0.0	0.0
Loans Held-for-sale, Fair Value Disclosure	0.0	0.0
Net loans held for investment	39.1	11.7
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	39.1	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
Federal Home Loan Bank Borrowings, Fair Value Disclosure	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