Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Sep. 30, 2024
Available-for-Sale:
Gross Unrealized Gains	$ 3.2	$ 6.6
Gross Unrealized Losses	(469.5)	(338.3)
Held-to-Maturity:
Investment securities classified as held to maturity	3,208.7	2,742.0
Allowance for Credit Losses	(0.8)	(0.7)
Net Amortized Cost	3,207.9	2,741.3
Gross Unrealized Gains	0.4	1.6
Gross Unrealized Losses	(334.3)	(262.4)
Investment debt securities held-to-maturity	2,874.0	2,480.5
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	10.7
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	(18.1)
Estimated Fair Value	5,841.5	5,534.3
Allowance for Credit Losses		0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	6,307.8	5,866.0
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value		1,478.0
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis		1,514.6
U.S. Treasury notes
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(25.5)	(19.2)
Held-to-Maturity:
Investment securities classified as held to maturity	399.0	99.6
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	399.0	99.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(2.8)	(0.4)
Investment debt securities held-to-maturity	396.2	99.2
Estimated Fair Value	224.7	231.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	250.2	250.2
State, county, and municipal securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(36.9)	(31.9)
Held-to-Maturity:
Investment securities classified as held to maturity	179.2	176.7
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	179.2	176.7
Gross Unrealized Gains	0.2	0.3
Gross Unrealized Losses	(24.2)	(20.6)
Investment debt securities held-to-maturity	155.2	156.4
Estimated Fair Value	219.8	220.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	256.7	252.8
Obligations of U.S. government agencies
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(10.9)	(6.5)
Held-to-Maturity:
Investment securities classified as held to maturity	354.5	356.4
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	354.5	356.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(42.4)	(32.0)
Investment debt securities held-to-maturity	312.1	324.4
Estimated Fair Value	168.5	165.4
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	179.4	171.9
Corporate Debt Securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(24.4)	(14.9)
Held-to-Maturity:
Investment securities classified as held to maturity	57.0	57.0
Allowance for Credit Losses	(0.8)	(0.7)
Net Amortized Cost	56.2	56.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(6.0)	(3.6)
Investment debt securities held-to-maturity	50.2	52.7
Estimated Fair Value	236.4	245.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	260.8	260.0
Collateralized Mortgage Obligations
Available-for-Sale:
Gross Unrealized Gains	1.3	2.2
Gross Unrealized Losses	(120.3)	(87.7)
Held-to-Maturity:
Investment securities classified as held to maturity	475.9	439.8
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	475.9	439.8
Gross Unrealized Gains	0.2	0.8
Gross Unrealized Losses	(69.0)	(57.8)
Investment debt securities held-to-maturity	407.1	382.8
Estimated Fair Value	1,189.5	1,116.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,308.5	1,202.1
Commercial Mortgage-Backed Securities
Available-for-Sale:
Gross Unrealized Gains	0.6	1.4
Gross Unrealized Losses	(87.7)	(60.2)
Held-to-Maturity:
Investment securities classified as held to maturity	510.5	493.0
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	510.5	493.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(52.9)	(41.8)
Investment debt securities held-to-maturity	457.6	451.2
Estimated Fair Value	1,105.6	1,105.3
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,192.7	1,164.1
Residential Mortgage Backed Securities
Available-for-Sale:
Gross Unrealized Gains	1.2	2.4
Gross Unrealized Losses	(130.6)	(94.2)
Held-to-Maturity:
Investment securities classified as held to maturity	1,232.6	1,119.5
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	1,232.6	1,119.5
Gross Unrealized Gains	0.0	0.5
Gross Unrealized Losses	(137.0)	(106.2)
Investment debt securities held-to-maturity	1,095.6	1,013.8
Estimated Fair Value	1,366.9	1,262.4
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,496.3	1,354.2
Private mortgage-backed securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(30.9)	(23.7)
Held-to-Maturity:
Estimated Fair Value	210.4	199.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	241.3	223.3
Collateralized Loan Obligations
Available-for-Sale:
Gross Unrealized Gains	0.1	0.6
Gross Unrealized Losses	(2.3)	0.0
Held-to-Maturity:
Estimated Fair Value	1,119.7	988.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	$ 1,121.9	$ 987.4