Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Financial assets:
Investment debt securities available-for-sale	$ 6,783.4	$ 4,820.5
Investment debt securities held-to-maturity	3,063.5	1,667.5
Derivative assets	42.7	28.1
Financial liabilities:
Derivative liabilities	169.9	18.2
Estimated Fair Value
Financial assets:
Cash and cash equivalents	591.9	2,344.8
Investment debt securities available-for-sale	6,783.4	4,820.5
Investment debt securities held-to-maturity	3,063.5	1,667.5
Accrued interest receivable	106.4	47.4
Mortgage servicing rights, net	36.6	28.2
Loans held for sale	93.6	30.1
Net loans held for investment	17,293.6	9,254.3
Derivative assets	42.7	28.1
Deferred compensation plan assets	19.0	21.4
Total financial assets	28,030.7	18,242.3
Financial liabilities:
Total deposits, excluding time deposits	24,255.6	15,303.1
Time deposits	1,587.2	963.1
Securities sold under repurchase agreements	1,075.6	1,051.1
Other Borrowings	625.0
Accrued interest payable	7.4	3.7
Long-term debt	116.7	120.7
Subordinated debentures held by subsidiary trusts	157.1	85.5
Derivative liabilities	169.9	18.2
Deferred compensation plan liabilities	19.0	21.4
Total financial liabilities	28,013.5	17,566.8
Carrying Amount
Financial assets:
Cash and cash equivalents	591.9	2,344.8
Investment debt securities available-for-sale	6,783.4	4,820.5
Investment debt securities held-to-maturity	3,485.7	1,687.6
Accrued interest receivable	106.4	47.4
Mortgage servicing rights, net	31.8	28.2
Loans held for sale	93.6	30.1
Net loans held for investment	17,390.5	9,209.4
Derivative assets	42.7	28.1
Deferred compensation plan assets	19.0	21.4
Total financial assets	28,545.0	18,217.5
Financial liabilities:
Total deposits, excluding time deposits	24,255.6	15,303.1
Time deposits	1,629.2	966.5
Securities sold under repurchase agreements	1,075.6	1,051.1
Other Borrowings	625.0
Accrued interest payable	7.4	3.7
Long-term debt	120.7	112.4
Subordinated debentures held by subsidiary trusts	163.1	87.0
Derivative liabilities	169.9	18.2
Deferred compensation plan liabilities	19.0	21.4
Total financial liabilities	28,065.5	17,563.4
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	591.9	2,344.8
Investment debt securities available-for-sale	336.8	684.7
Investment debt securities held-to-maturity	385.5	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	1,314.2	3,029.5
Financial liabilities:
Total deposits, excluding time deposits	24,255.6	15,303.1
Time deposits	0.0	0.0
Securities sold under repurchase agreements	0.0	0.0
Other Borrowings	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	24,255.6	15,303.1
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Investment debt securities available-for-sale	6,446.6	4,135.8
Investment debt securities held-to-maturity	2,678.0	1,667.5
Accrued interest receivable	106.4	47.4
Mortgage servicing rights, net	36.6	28.2
Loans held for sale	93.6	30.1
Net loans held for investment	17,236.0	9,242.6
Derivative assets	42.7	28.1
Deferred compensation plan assets	19.0	21.4
Total financial assets	26,658.9	15,201.1
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,587.2	963.1
Securities sold under repurchase agreements	1,075.6	1,051.1
Other Borrowings	625.0
Accrued interest payable	7.4	3.7
Long-term debt	116.7	120.7
Subordinated debentures held by subsidiary trusts	157.1	85.5
Derivative liabilities	169.9	18.2
Deferred compensation plan liabilities	19.0	21.4
Total financial liabilities	3,757.9	2,263.7
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	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	57.6	11.7
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	57.6	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
Other Borrowings	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