Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2025	Dec. 31, 2024
Available-for-Sale:
Fair Value, Less than 12 Months	$ 111.2	$ 133.4
Gross Unrealized Losses, Less than 12 Months	(0.5)	(2.7)
Fair Value, 12 Months or Longer	3,727.6	4,047.2
Gross Unrealized Losses, 12 Months or Longer	(269.9)	(417.4)
Fair Value, Total	3,838.8	4,180.6
Gross Unrealized Losses, Total	(270.4)	(420.1)
Allowance for Credit Losses	0.0
Gross Unrealized Gains	6.3	2.4
Gross Unrealized Losses	(270.4)	(420.1)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	4,895.5	5,057.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	5,159.6	5,474.8
Investment securities classified as held to maturity	2,411.0	2,688.4
Allowance for Credit Losses	(0.7)	(0.9)
Net Amortized Cost	2,410.3	2,687.5
Gross Unrealized Gains	1.0	0.2
Gross Unrealized Losses	(226.6)	(330.0)
Investment debt securities held-to-maturity	2,185.4	2,358.6
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss		15.9
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain		(8.0)
US Treasury Notes Securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.0
Gross Unrealized Losses, Less than 12 Months	0.0
Fair Value, 12 Months or Longer	235.8
Gross Unrealized Losses, 12 Months or Longer	(10.3)
Fair Value, Total	235.8
Gross Unrealized Losses, Total	(10.3)
U.S. Treasury notes
Available-for-Sale:
Fair Value, Less than 12 Months		0.0
Gross Unrealized Losses, Less than 12 Months		0.0
Fair Value, 12 Months or Longer		226.8
Gross Unrealized Losses, 12 Months or Longer		(18.1)
Fair Value, Total		226.8
Gross Unrealized Losses, Total		(18.1)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(10.3)	(18.1)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	235.8	226.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	246.1	245.0
Investment securities classified as held to maturity	0.0	99.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.4)
Investment debt securities held-to-maturity	0.0	99.4
State, county, and municipal securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	0.0
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	216.5	210.1
Gross Unrealized Losses, 12 Months or Longer	(30.4)	(40.0)
Fair Value, Total	216.5	210.1
Gross Unrealized Losses, Total	(30.4)	(40.0)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(30.4)	(40.0)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	218.2	212.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	248.6	252.0
Investment securities classified as held to maturity	175.1	176.3
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	(19.8)	(26.7)
Investment debt securities held-to-maturity	155.5	149.8
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	3.1	3.6
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	195.7	206.2
Gross Unrealized Losses, 12 Months or Longer	(5.2)	(10.8)
Fair Value, Total	198.8	209.8
Gross Unrealized Losses, Total	(5.2)	(10.8)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(5.2)	(10.8)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	201.9	213.3
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	207.1	224.1
Investment securities classified as held to maturity	464.7	470.1
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(40.7)	(59.9)
Investment debt securities held-to-maturity	424.1	410.2
Private mortgage-backed securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	0.0
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	177.9	190.4
Gross Unrealized Losses, 12 Months or Longer	(21.2)	(27.6)
Fair Value, Total	177.9	190.4
Gross Unrealized Losses, Total	(21.2)	(27.6)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(21.2)	(27.6)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	178.0	190.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	199.2	218.1
Collateralized Loan Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	30.0	10.0
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	0.0	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0	0.0
Fair Value, Total	30.0	10.0
Gross Unrealized Losses, Total	0.0	0.0
Gross Unrealized Gains	0.9	1.3
Gross Unrealized Losses	0.0	0.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	671.9	772.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	671.0	770.7
Corporate Debt Securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	27.5
Gross Unrealized Losses, Less than 12 Months	0.0	(0.5)
Fair Value, 12 Months or Longer	153.3	205.5
Gross Unrealized Losses, 12 Months or Longer	(9.7)	(16.2)
Fair Value, Total	153.3	233.0
Gross Unrealized Losses, Total	(9.7)	(16.7)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(9.7)	(16.7)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	162.3	232.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	172.0	249.6
Investment securities classified as held to maturity	39.9	57.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(1.3)	(2.8)
Investment debt securities held-to-maturity	38.6	54.2
Commercial Mortgage-Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	38.1
Gross Unrealized Losses, Less than 12 Months	0.0	(0.4)
Fair Value, 12 Months or Longer	866.4	934.9
Gross Unrealized Losses, 12 Months or Longer	(42.4)	(74.1)
Fair Value, Total	866.4	973.0
Gross Unrealized Losses, Total	(42.4)	(74.5)
Gross Unrealized Gains	1.0	0.1
Gross Unrealized Losses	(42.4)	(74.5)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	900.3	982.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	941.7	1,057.2
Investment securities classified as held to maturity	363.1	374.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(21.7)	(33.4)
Investment debt securities held-to-maturity	341.4	341.5
Residential Mortgage Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	71.5	33.7
Gross Unrealized Losses, Less than 12 Months	(0.5)	(1.5)
Fair Value, 12 Months or Longer	980.9	1,086.2
Gross Unrealized Losses, 12 Months or Longer	(78.2)	(124.3)
Fair Value, Total	1,052.4	1,119.9
Gross Unrealized Losses, Total	(78.7)	(125.8)
Gross Unrealized Gains	3.1	0.4
Gross Unrealized Losses	(78.7)	(125.8)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,370.8	1,175.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,446.4	1,301.2
Investment securities classified as held to maturity	976.4	1,082.5
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(94.4)	(140.7)
Investment debt securities held-to-maturity	882.1	941.8
Collateralized Mortgage Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	6.6	20.5
Gross Unrealized Losses, Less than 12 Months	0.0	(0.3)
Fair Value, 12 Months or Longer	901.1	987.1
Gross Unrealized Losses, 12 Months or Longer	(72.5)	(106.3)
Fair Value, Total	907.7	1,007.6
Gross Unrealized Losses, Total	(72.5)	(106.6)
Gross Unrealized Gains	1.3	0.6
Gross Unrealized Losses	(72.5)	(106.6)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	956.3	1,050.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,027.5	1,156.9
Investment securities classified as held to maturity	391.8	427.8
Gross Unrealized Gains	0.6	0.0
Gross Unrealized Losses	(48.7)	(66.1)
Investment debt securities held-to-maturity	343.7	$ 361.7
US agency, residential and commercial MBS and CMOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	(6.4)
State, county & municipal, obligations of US govt agencies and CLOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	$ 14.3

Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain

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