Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Mar. 31, 2026
Available-for-Sale:
Gross Unrealized Gains	$ 8.1	$ 3.7
Gross Unrealized Losses	(238.0)	(257.1)
Held-to-Maturity:
Investment securities classified as held to maturity	2,342.6	2,293.9
Allowance for Credit Losses	(0.5)	(0.5)
Net Amortized Cost	2,342.1	2,293.4
Gross Unrealized Gains	1.0	0.5
Gross Unrealized Losses	(207.0)	(211.7)
Investment debt securities held-to-maturity	2,136.6	2,082.7
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	5.9
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	(13.7)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	5,288.1	5,716.6
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	5,518.0	5,970.0
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value		604.9
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis		631.7
U.S. Treasury notes
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(8.8)	(9.4)
Held-to-Maturity:
Debt Securities, Available-for-Sale, Excluding Accrued Interest	237.7	237.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	246.5	246.9
State, county, and municipal securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(27.2)	(29.6)
Held-to-Maturity:
Investment securities classified as held to maturity	173.2	173.1
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Losses	(18.6)	(19.6)
Investment debt securities held-to-maturity	154.8	153.6
Debt Securities, Available-for-Sale, Excluding Accrued Interest	220.1	217.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	247.3	246.7
Obligations of U.S. government agencies
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(4.2)	(4.4)
Held-to-Maturity:
Investment securities classified as held to maturity	462.3	458.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(37.7)	(38.5)
Investment debt securities held-to-maturity	424.6	420.1
Debt Securities, Available-for-Sale, Excluding Accrued Interest	200.9	175.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	205.1	180.3
Corporate Debt Securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(8.7)	(8.2)
Held-to-Maturity:
Investment securities classified as held to maturity	29.9	29.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.8)	(0.7)
Investment debt securities held-to-maturity	29.1	29.2
Debt Securities, Available-for-Sale, Excluding Accrued Interest	150.0	140.3
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	158.7	148.5
Collateralized Mortgage Obligations
Available-for-Sale:
Gross Unrealized Gains	1.4	0.8
Gross Unrealized Losses	(64.8)	(68.9)
Held-to-Maturity:
Investment securities classified as held to maturity	381.1	370.5
Gross Unrealized Gains	0.7	0.4
Gross Unrealized Losses	(46.4)	(48.1)
Investment debt securities held-to-maturity	335.4	322.8
Debt Securities, Available-for-Sale, Excluding Accrued Interest	922.7	880.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	986.1	949.0
Commercial Mortgage-Backed Securities
Available-for-Sale:
Gross Unrealized Gains	1.1	0.9
Gross Unrealized Losses	(36.1)	(36.7)
Held-to-Maturity:
Investment securities classified as held to maturity	354.8	353.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(18.8)	(19.3)
Investment debt securities held-to-maturity	336.0	334.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	856.7	839.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	891.7	875.7
Residential Mortgage Backed Securities
Available-for-Sale:
Gross Unrealized Gains	4.4	1.9
Gross Unrealized Losses	(68.7)	(79.8)
Held-to-Maturity:
Investment securities classified as held to maturity	941.3	908.5
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(84.7)	(85.5)
Investment debt securities held-to-maturity	856.7	823.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,770.1	2,223.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,834.4	2,301.0
Private mortgage-backed securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(19.5)	(19.6)
Held-to-Maturity:
Debt Securities, Available-for-Sale, Excluding Accrued Interest	174.4	168.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	193.9	188.1
Collateralized Loan Obligations
Available-for-Sale:
Gross Unrealized Gains	1.2	0.1
Gross Unrealized Losses	0.0	(0.5)
Held-to-Maturity:
Debt Securities, Available-for-Sale, Excluding Accrued Interest	755.5	833.4
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	$ 754.3	$ 833.8