Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Jun. 30, 2025	Dec. 31, 2024
Available-for-Sale:
Gross Unrealized Gains		$ 4.6	$ 2.4
Gross Unrealized Losses		(306.9)	(420.1)
Held-to-Maturity:
Investment securities classified as held to maturity		2,570.7	2,688.4
Allowance for Credit Losses		(0.7)	(0.9)
Net Amortized Cost		2,570.0	2,687.5
Gross Unrealized Gains		0.9	0.2
Gross Unrealized Losses		(254.6)	(330.0)
Investment debt securities held-to-maturity		2,317.0	2,358.6
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		4,742.2	5,057.1
Allowance for Credit Losses		0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss		5,044.5	5,474.8
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value		1,031.2
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis		1,064.8
U.S. Treasury notes
Available-for-Sale:
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(11.8)	(18.1)
Held-to-Maturity:
Investment securities classified as held to maturity		100.0	99.8
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(0.1)	(0.4)
Investment debt securities held-to-maturity		99.9	99.4
Debt Securities, Available-for-Sale, Excluding Accrued Interest		233.9	226.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss		245.7	245.0
State, county, and municipal securities
Available-for-Sale:
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(35.8)	(40.0)
Held-to-Maturity:
Investment securities classified as held to maturity		175.8	176.3
Gross Unrealized Gains		0.2	0.2
Gross Unrealized Losses		(23.5)	(26.7)
Investment debt securities held-to-maturity		152.5	149.8
Debt Securities, Available-for-Sale, Excluding Accrued Interest		213.4	212.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss		249.2	252.0
Obligations of U.S. government agencies
Available-for-Sale:
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(6.5)	(10.8)
Held-to-Maturity:
Investment securities classified as held to maturity		466.8	470.1
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(45.2)	(59.9)
Investment debt securities held-to-maturity		421.6	410.2
Debt Securities, Available-for-Sale, Excluding Accrued Interest		206.7	213.3
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss		213.2	224.1
Corporate Debt Securities
Available-for-Sale:
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(12.0)	(16.7)
Held-to-Maturity:
Investment securities classified as held to maturity		40.0	57.0
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(1.8)	(2.8)
Investment debt securities held-to-maturity		38.2	54.2
Debt Securities, Available-for-Sale, Excluding Accrued Interest		227.0	232.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss		239.0	249.6
Collateralized Mortgage Obligations
Available-for-Sale:
Gross Unrealized Gains		1.4	0.6
Gross Unrealized Losses		(80.0)	(106.6)
Held-to-Maturity:
Investment securities classified as held to maturity		405.0	427.8
Gross Unrealized Gains		0.5	0.0
Gross Unrealized Losses		(54.1)	(66.1)
Investment debt securities held-to-maturity		351.4	361.7
Debt Securities, Available-for-Sale, Excluding Accrued Interest		993.6	1,050.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss		1,072.2	1,156.9
Commercial Mortgage-Backed Securities
Available-for-Sale:
Gross Unrealized Gains		1.0	0.1
Gross Unrealized Losses		(49.6)	(74.5)
Held-to-Maturity:
Investment securities classified as held to maturity		369.7	374.9
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(24.8)	(33.4)
Investment debt securities held-to-maturity		344.9	341.5
Debt Securities, Available-for-Sale, Excluding Accrued Interest		947.1	982.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss		995.7	1,057.2
Residential Mortgage Backed Securities
Available-for-Sale:
Gross Unrealized Gains		1.2	0.4
Gross Unrealized Losses		(88.0)	(125.8)
Held-to-Maturity:
Investment securities classified as held to maturity		1,013.4	1,082.5
Gross Unrealized Gains		0.2	0.0
Gross Unrealized Losses		(105.1)	(140.7)
Investment debt securities held-to-maturity		908.5	941.8
Debt Securities, Available-for-Sale, Excluding Accrued Interest		1,111.4	1,175.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss		1,198.2	1,301.2
Private mortgage-backed securities
Available-for-Sale:
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(23.2)	(27.6)
Held-to-Maturity:
Debt Securities, Available-for-Sale, Excluding Accrued Interest		182.3	190.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss		205.5	218.1
Collateralized Loan Obligations
Available-for-Sale:
Gross Unrealized Gains		1.0	1.3
Gross Unrealized Losses		0.0	0.0
Held-to-Maturity:
Debt Securities, Available-for-Sale, Excluding Accrued Interest		626.8	772.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss		$ 625.8	$ 770.7

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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