Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023
Schedule of Available for Sale and Held-to-Maturity Securities
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost	$ 64.7
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	1,292.9
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value	1,247.2
Available-for-Sale:
Gross Unrealized Gains	2.4	$ 3.2
Gross Unrealized Losses	(420.1)	(469.5)
Held-to-Maturity:
Debt Securities, Held-to-maturity	2,688.4	3,208.7
Allowance for Credit Losses	0.2	0.4
Gross Unrealized Losses	(330.0)	(335.1)
Held-to-maturity investment securities, estimated fair value	2,358.6	2,874.0
Debt Securities, Held-to-Maturity, Transfer, Unamortized Gain	(8.0)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	63.9
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	101.2
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	100.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,338.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	1,258.9
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	435.9
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	408.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	1,131.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	1,048.9
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	471.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	413.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	2,940.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	2,685.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	1,680.0
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	1,435.8
Debt Securities, Held-to-maturity, Allowance for Credit Loss	(0.9)	(0.8)
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	2,687.5	3,207.9
Debt and Equity Securities, Unrealized Gain (Loss)	15.9	10.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	5,474.8	6,307.8
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	5,057.1	5,841.5
US Treasury notes
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(18.1)	(25.5)
Held-to-Maturity:
Debt Securities, Held-to-maturity	99.8	399.0
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(0.4)	(2.8)
Held-to-maturity investment securities, estimated fair value	99.4	396.2
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	245.0	250.2
Debt Securities, Available-for-Sale, Excluding Accrued Interest	226.9	224.7
State, county and municipal securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(40.0)	(36.9)
Held-to-Maturity:
Debt Securities, Held-to-maturity	176.3	179.2
Allowance for Credit Losses	0.2	0.2
Gross Unrealized Losses	(26.7)	(24.2)
Held-to-maturity investment securities, estimated fair value	149.8	155.2
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	252.0	256.7
Debt Securities, Available-for-Sale, Excluding Accrued Interest	212.0	219.8
US Government Agencies Debt Securities [Member]
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(10.8)	(10.9)
Held-to-Maturity:
Debt Securities, Held-to-maturity	470.1	354.5
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(59.9)	(42.4)
Held-to-maturity investment securities, estimated fair value	410.2	312.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	224.1	179.4
Debt Securities, Available-for-Sale, Excluding Accrued Interest	213.3	168.5
Corporate Debt Securities [Member]
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(16.7)	(24.4)
Held-to-Maturity:
Debt Securities, Held-to-maturity	57.0	57.0
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(2.8)	(6.8)
Held-to-maturity investment securities, estimated fair value	54.2	50.2
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	249.6	260.8
Debt Securities, Available-for-Sale, Excluding Accrued Interest	232.9	236.4
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(27.6)	(30.9)
Held-to-Maturity:
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	218.1	241.3
Debt Securities, Available-for-Sale, Excluding Accrued Interest	190.5	210.4
Collateralized Loan Obligations
Available-for-Sale:
Gross Unrealized Gains	1.3	0.1
Gross Unrealized Losses	0.0	(2.3)
Held-to-Maturity:
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	770.7	1,121.9
Debt Securities, Available-for-Sale, Excluding Accrued Interest	772.0	1,119.7
Residential Mortgage-Backed Securities
Available-for-Sale:
Gross Unrealized Gains	0.4	1.2
Gross Unrealized Losses	(125.8)	(130.6)
Held-to-Maturity:
Debt Securities, Held-to-maturity	1,082.5	1,232.6
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(140.7)	(137.0)
Held-to-maturity investment securities, estimated fair value	941.8	1,095.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,301.2	1,496.3
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,175.8	1,366.9
Collateralized Mortgage Obligations
Available-for-Sale:
Gross Unrealized Gains	0.6	1.3
Gross Unrealized Losses	(106.6)	(120.3)
Held-to-Maturity:
Debt Securities, Held-to-maturity	427.8	475.9
Allowance for Credit Losses	0.0	0.2
Gross Unrealized Losses	(66.1)	(69.0)
Held-to-maturity investment securities, estimated fair value	361.7	407.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,156.9	1,308.5
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,050.9	1,189.5
Commercial Mortgage Backed Securities
Available-for-Sale:
Gross Unrealized Gains	0.1	0.6
Gross Unrealized Losses	(74.5)	(87.7)
Held-to-Maturity:
Debt Securities, Held-to-maturity	374.9	510.5
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(33.4)	(52.9)
Held-to-maturity investment securities, estimated fair value	341.5	457.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,057.2	1,192.7
Debt Securities, Available-for-Sale, Excluding Accrued Interest	$ 982.8	1,105.6
US agency, residential and commercial MBS and CMOs
Held-to-Maturity:
Debt Securities, Held-to-Maturity, Transfer, Unamortized Gain		$ (18.1)

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