Investment Securities - Credit Quality Indicators (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	$ 2,293.9	$ 2,342.6
State, county, and municipal securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	173.1	173.2
Obligations of U.S. government agencies
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	458.6	462.3
Corporate securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	29.9	29.9
Commercial Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	353.3	354.8
Residential Mortgage Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	908.5	941.3
Collateralized Mortgage Obligations
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	$ 370.5	$ 381.1