Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
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	$ 114.4
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	603.6
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value	577.8
Available-for-Sale:
Gross Unrealized Gains	8.1	$ 2.4
Gross Unrealized Losses	(238.0)	(420.1)
Held-to-Maturity:
Debt Securities, Held-to-maturity	2,342.6	2,688.4
Allowance for Credit Losses	1.0	0.2
Gross Unrealized Losses	(207.0)	(330.0)
Held-to-maturity investment securities, estimated fair value	2,136.6	2,358.6
Debt Securities, Held-to-Maturity, Transfer, Unamortized Gain	(5.9)	(15.9)
Debt and Equity Securities, Unrealized Gain (Loss)	13.7	8.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	113.5
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	32.8
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	32.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,239.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	1,192.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	617.0
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	587.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	944.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	877.5
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	273.2
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	249.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	3,219.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	3,104.5
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	1,419.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	1,266.8
Debt Securities, Held-to-maturity, Allowance for Credit Loss	(0.5)	(0.9)
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	2,342.1	2,687.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	5,518.0	5,474.8
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0	0.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	5,288.1	5,057.1
US Treasury notes
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(8.8)	(18.1)
Held-to-Maturity:
Debt Securities, Held-to-maturity		99.8
Allowance for Credit Losses		0.0
Gross Unrealized Losses		(0.4)
Held-to-maturity investment securities, estimated fair value		99.4
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	246.5	245.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	237.7	226.9
State, county and municipal securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(27.2)	(40.0)
Held-to-Maturity:
Debt Securities, Held-to-maturity	173.2	176.3
Allowance for Credit Losses	0.2	0.2
Gross Unrealized Losses	(18.6)	(26.7)
Held-to-maturity investment securities, estimated fair value	154.8	149.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	247.3	252.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	220.1	212.0
US Government Agencies Debt Securities [Member]
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(4.2)	(10.8)
Held-to-Maturity:
Debt Securities, Held-to-maturity	462.3	470.1
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(37.7)	(59.9)
Held-to-maturity investment securities, estimated fair value	424.6	410.2
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	205.1	224.1
Debt Securities, Available-for-Sale, Excluding Accrued Interest	200.9	213.3
Corporate Debt Securities [Member]
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(8.7)	(16.7)
Held-to-Maturity:
Debt Securities, Held-to-maturity	29.9	57.0
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(0.8)	(2.8)
Held-to-maturity investment securities, estimated fair value	29.1	54.2
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	158.7	249.6
Debt Securities, Available-for-Sale, Excluding Accrued Interest	150.0	232.9
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(19.5)	(27.6)
Held-to-Maturity:
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	193.9	218.1
Debt Securities, Available-for-Sale, Excluding Accrued Interest	174.4	190.5
Collateralized Loan Obligations
Available-for-Sale:
Gross Unrealized Gains	1.2	1.3
Gross Unrealized Losses	0.0	0.0
Held-to-Maturity:
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	754.3	770.7
Debt Securities, Available-for-Sale, Excluding Accrued Interest	755.5	772.0
Residential Mortgage-Backed Securities
Available-for-Sale:
Gross Unrealized Gains	4.4	0.4
Gross Unrealized Losses	(68.7)	(125.8)
Held-to-Maturity:
Debt Securities, Held-to-maturity	941.3	1,082.5
Allowance for Credit Losses	0.1	0.0
Gross Unrealized Losses	(84.7)	(140.7)
Held-to-maturity investment securities, estimated fair value	856.7	941.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,834.4	1,301.2
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,770.1	1,175.8
Collateralized Mortgage Obligations
Available-for-Sale:
Gross Unrealized Gains	1.4	0.6
Gross Unrealized Losses	(64.8)	(106.6)
Held-to-Maturity:
Debt Securities, Held-to-maturity	381.1	427.8
Allowance for Credit Losses	0.7	0.0
Gross Unrealized Losses	(46.4)	(66.1)
Held-to-maturity investment securities, estimated fair value	335.4	361.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	986.1	1,156.9
Debt Securities, Available-for-Sale, Excluding Accrued Interest	922.7	1,050.9
Commercial Mortgage Backed Securities
Available-for-Sale:
Gross Unrealized Gains	1.1	0.1
Gross Unrealized Losses	(36.1)	(74.5)
Held-to-Maturity:
Debt Securities, Held-to-maturity	354.8	374.9
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(18.8)	(33.4)
Held-to-maturity investment securities, estimated fair value	336.0	341.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	891.7	1,057.2
Debt Securities, Available-for-Sale, Excluding Accrued Interest	$ 856.7	$ 982.8

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