Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Financial assets:
Securities Purchased under Agreements to Resell	$ 202.2	$ 0.0
Investment debt securities available-for-sale	7,741.1	4,820.5
Investment debt securities held-to-maturity	2,860.5	1,667.5
Derivative assets	24.8	28.1
Financial liabilities:
Derivative liabilities	99.4	18.2
Carrying Amount
Financial assets:
Cash and cash equivalents	1,059.3	2,344.8
Securities Purchased under Agreements to Resell	202.2
Investment debt securities available-for-sale	7,741.1	4,820.5
Investment debt securities held-to-maturity	3,130.0	1,687.6
Accrued interest receivable	90.6	47.4
Mortgage servicing rights, net	32.1	28.2
Loans held for sale	127.4	30.1
Net loans held for investment	16,942.1	9,209.4
Derivative assets	24.8	28.1
Deferred compensation plan assets	19.3	21.4
Total financial assets	29,368.9	18,217.5
Financial liabilities:
Total deposits, excluding time deposits	25,368.1	15,303.1
Time deposits	1,495.7	966.5
Securities sold under repurchase agreements	1,234.7	1,051.1
Accrued interest payable	4.3	3.7
Long-term debt	120.4	112.4
Subordinated debentures held by subsidiary trusts	163.1	87.0
Derivative liabilities	99.4	18.2
Deferred compensation plan liabilities	19.3	21.4
Total financial liabilities	28,505.0	17,563.4
Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,059.3	2,344.8
Securities Purchased under Agreements to Resell	202.2
Investment debt securities available-for-sale	7,741.1	4,820.5
Investment debt securities held-to-maturity	2,860.5	1,667.5
Accrued interest receivable	90.6	47.4
Mortgage servicing rights, net	37.0	28.2
Loans held for sale	127.4	30.1
Net loans held for investment	16,874.0	9,254.3
Derivative assets	24.8	28.1
Deferred compensation plan assets	19.3	21.4
Total financial assets	29,036.2	18,242.3
Financial liabilities:
Total deposits, excluding time deposits	25,368.1	15,303.1
Time deposits	1,467.2	963.1
Securities sold under repurchase agreements	1,234.7	1,051.1
Accrued interest payable	4.3	3.7
Long-term debt	119.3	120.7
Subordinated debentures held by subsidiary trusts	157.5	85.5
Derivative liabilities	99.4	18.2
Deferred compensation plan liabilities	19.3	21.4
Total financial liabilities	28,469.8	17,566.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,059.3	2,344.8
Securities Purchased under Agreements to Resell	0.0
Investment debt securities available-for-sale	809.8	684.7
Investment debt securities held-to-maturity	293.1	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	2,162.2	3,029.5
Financial liabilities:
Total deposits, excluding time deposits	25,368.1	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	25,368.1	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	202.2
Investment debt securities available-for-sale	6,931.3	4,135.8
Investment debt securities held-to-maturity	2,567.4	1,667.5
Accrued interest receivable	90.6	47.4
Mortgage servicing rights, net	37.0	28.2
Loans held for sale	127.4	30.1
Net loans held for investment	16,788.5	9,242.6
Derivative assets	24.8	28.1
Deferred compensation plan assets	19.3	21.4
Total financial assets	26,788.5	15,201.1
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,467.2	963.1
Securities sold under repurchase agreements	1,234.7	1,051.1
Accrued interest payable	4.3	3.7
Long-term debt	119.3	120.7
Subordinated debentures held by subsidiary trusts	157.5	85.5
Derivative liabilities	99.4	18.2
Deferred compensation plan liabilities	19.3	21.4
Total financial liabilities	3,101.7	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	85.5	11.7
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	85.5	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