Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Mar. 31, 2025
Available-for-Sale:
Gross Unrealized Gains	$ 2.4	$ 2.7
Gross Unrealized Losses	(420.1)	(349.7)
Held-to-Maturity:
Investment securities classified as held to maturity	2,688.4	2,640.3
Allowance for Credit Losses	(0.9)	(0.8)
Net Amortized Cost	2,687.5	2,639.5
Gross Unrealized Gains	0.2	0.7
Gross Unrealized Losses	(330.0)	(279.8)
Investment debt securities held-to-maturity	2,358.6	2,361.2
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	8.0
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	(15.9)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	5,057.1	4,864.3
Allowance for Credit Losses		0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	5,474.8	5,211.3
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value		1,043.2
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis		1,081.3
U.S. Treasury notes
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(18.1)	(14.1)
Held-to-Maturity:
Investment securities classified as held to maturity	99.8	99.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.4)	(0.3)
Investment debt securities held-to-maturity	99.4	99.5
Debt Securities, Available-for-Sale, Excluding Accrued Interest	226.9	231.2
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	245.0	245.3
State, county, and municipal securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(40.0)	(37.8)
Held-to-Maturity:
Investment securities classified as held to maturity	176.3	176.2
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	(26.7)	(24.2)
Investment debt securities held-to-maturity	149.8	152.2
Debt Securities, Available-for-Sale, Excluding Accrued Interest	212.0	213.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	252.0	251.5
Obligations of U.S. government agencies
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(10.8)	(7.9)
Held-to-Maturity:
Investment securities classified as held to maturity	470.1	468.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(59.9)	(50.0)
Investment debt securities held-to-maturity	410.2	418.2
Debt Securities, Available-for-Sale, Excluding Accrued Interest	213.3	207.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	224.1	215.4
Corporate Debt Securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(16.7)	(13.5)
Held-to-Maturity:
Investment securities classified as held to maturity	57.0	56.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(2.8)	(1.9)
Investment debt securities held-to-maturity	54.2	55.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	232.9	225.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	249.6	239.3
Collateralized Mortgage Obligations
Available-for-Sale:
Gross Unrealized Gains	0.6	1.1
Gross Unrealized Losses	(106.6)	(89.9)
Held-to-Maturity:
Investment securities classified as held to maturity	427.8	417.3
Gross Unrealized Gains	0.0	0.4
Gross Unrealized Losses	(66.1)	(58.4)
Investment debt securities held-to-maturity	361.7	359.3
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,050.9	1,028.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,156.9	1,117.4
Commercial Mortgage-Backed Securities
Available-for-Sale:
Gross Unrealized Gains	0.1	0.5
Gross Unrealized Losses	(74.5)	(59.4)
Held-to-Maturity:
Investment securities classified as held to maturity	374.9	372.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(33.4)	(28.0)
Investment debt securities held-to-maturity	341.5	344.7
Debt Securities, Available-for-Sale, Excluding Accrued Interest	982.8	986.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,057.2	1,045.6
Residential Mortgage Backed Securities
Available-for-Sale:
Gross Unrealized Gains	0.4	0.9
Gross Unrealized Losses	(125.8)	(101.6)
Held-to-Maturity:
Investment securities classified as held to maturity	1,082.5	1,049.2
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	(140.7)	(117.0)
Investment debt securities held-to-maturity	941.8	932.3
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,175.8	1,149.3
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,301.2	1,250.0
Private mortgage-backed securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(27.6)	(24.5)
Held-to-Maturity:
Debt Securities, Available-for-Sale, Excluding Accrued Interest	190.5	187.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	218.1	212.1
Collateralized Loan Obligations
Available-for-Sale:
Gross Unrealized Gains	1.3	0.2
Gross Unrealized Losses	0.0	(1.0)
Held-to-Maturity:
Debt Securities, Available-for-Sale, Excluding Accrued Interest	772.0	633.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	$ 770.7	$ 634.7