Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2026	Dec. 31, 2025
Available-for-Sale:
Fair Value, Less than 12 Months	$ 1,458.9	$ 258.1
Gross Unrealized Losses, Less than 12 Months	(16.9)	(1.1)
Fair Value, 12 Months or Longer	3,294.6	3,616.2
Gross Unrealized Losses, 12 Months or Longer	(250.0)	(236.9)
Fair Value, Total	4,753.5	3,874.3
Gross Unrealized Losses, Total	(266.9)	(238.0)
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Gains	3.1	8.1
Gross Unrealized Losses	(266.9)	(238.0)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	5,769.8	5,288.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	6,033.6	5,518.0
Investment securities classified as held to maturity	2,215.9	2,342.6
Allowance for Credit Losses	(0.1)	(0.5)
Amortized Cost, Net(1)	2,215.8	2,342.1
Gross Unrealized Gains	0.4	1.0
Gross Unrealized Losses	(216.0)	(207.0)
Investment debt securities held-to-maturity	2,000.3	2,136.6
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss		13.7
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain		(5.9)
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	237.5
Gross Unrealized Losses, 12 Months or Longer	(9.8)
Fair Value, Total	237.5
Gross Unrealized Losses, Total	(9.8)
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		237.7
Gross Unrealized Losses, 12 Months or Longer		(8.8)
Fair Value, Total		237.7
Gross Unrealized Losses, Total		(8.8)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(9.8)	(8.8)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	237.5	237.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	247.3	246.5
State, county, and municipal securities
Available-for-Sale:
Fair Value, Less than 12 Months	8.7	0.0
Gross Unrealized Losses, Less than 12 Months	(0.1)	0.0
Fair Value, 12 Months or Longer	207.2	210.4
Gross Unrealized Losses, 12 Months or Longer	(27.5)	(27.2)
Fair Value, Total	215.9	210.4
Gross Unrealized Losses, Total	(27.6)	(27.2)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(27.6)	(27.2)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	216.9	220.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	244.5	247.3
Investment securities classified as held to maturity	164.9	173.2
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	(19.4)	(18.6)
Investment debt securities held-to-maturity	145.6	154.8
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	2.4
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	167.4	195.8
Gross Unrealized Losses, 12 Months or Longer	(4.7)	(4.2)
Fair Value, Total	167.4	198.2
Gross Unrealized Losses, Total	(4.7)	(4.2)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(4.7)	(4.2)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	171.8	200.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	176.5	205.1
Investment securities classified as held to maturity	455.9	462.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(39.9)	(37.7)
Investment debt securities held-to-maturity	416.0	424.6
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	161.0	174.3
Gross Unrealized Losses, 12 Months or Longer	(19.9)	(19.5)
Fair Value, Total	161.0	174.3
Gross Unrealized Losses, Total	(19.9)	(19.5)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(19.9)	(19.5)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	161.1	174.4
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	181.0	193.9
Collateralized Loan Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	144.9
Gross Unrealized Losses, Less than 12 Months	(0.1)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	144.9
Gross Unrealized Losses, Total	(0.1)
Gross Unrealized Gains	0.8	1.2
Gross Unrealized Losses	(0.1)	0.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	809.5	755.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	808.8	754.3
Corporate Debt Securities
Available-for-Sale:
Fair Value, Less than 12 Months	12.4	0.0
Gross Unrealized Losses, Less than 12 Months	(0.1)	0.0
Fair Value, 12 Months or Longer	97.8	150.0
Gross Unrealized Losses, 12 Months or Longer	(8.0)	(8.7)
Fair Value, Total	110.2	150.0
Gross Unrealized Losses, Total	(8.1)	(8.7)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(8.1)	(8.7)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	110.2	150.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	118.3	158.7
Investment securities classified as held to maturity	10.9	29.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	(0.8)
Investment debt securities held-to-maturity	10.8	29.1
Commercial Mortgage-Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	1.3
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	789.3	821.6
Gross Unrealized Losses, 12 Months or Longer	(37.6)	(36.1)
Fair Value, Total	789.3	822.9
Gross Unrealized Losses, Total	(37.6)	(36.1)
Gross Unrealized Gains	0.7	1.1
Gross Unrealized Losses	(37.6)	(36.1)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	822.7	856.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	859.6	891.7
Investment securities classified as held to maturity	351.8	354.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(20.1)	(18.8)
Investment debt securities held-to-maturity	331.7	336.0
Residential Mortgage Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	1,274.9	254.4
Gross Unrealized Losses, Less than 12 Months	(16.4)	(1.1)
Fair Value, 12 Months or Longer	845.8	956.5
Gross Unrealized Losses, 12 Months or Longer	(72.9)	(67.6)
Fair Value, Total	2,120.7	1,210.9
Gross Unrealized Losses, Total	(89.3)	(68.7)
Gross Unrealized Gains	1.0	4.4
Gross Unrealized Losses	(89.3)	(68.7)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	2,402.9	1,770.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	2,491.2	1,834.4
Investment securities classified as held to maturity	873.9	941.3
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	(88.6)	(84.7)
Investment debt securities held-to-maturity	785.3	856.7
Collateralized Mortgage Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	18.0	0.0
Gross Unrealized Losses, Less than 12 Months	(0.2)	0.0
Fair Value, 12 Months or Longer	788.6	869.9
Gross Unrealized Losses, 12 Months or Longer	(69.6)	(64.8)
Fair Value, Total	806.6	869.9
Gross Unrealized Losses, Total	(69.8)	(64.8)
Gross Unrealized Gains	0.6	1.4
Gross Unrealized Losses	(69.8)	(64.8)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	837.2	922.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	906.4	986.1
Investment securities classified as held to maturity	358.5	381.1
Gross Unrealized Gains	0.3	0.7
Gross Unrealized Losses	(47.9)	(46.4)
Investment debt securities held-to-maturity	310.9	$ 335.4
US agency, residential and commercial MBS and CMOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	(5.1)
State, county & municipal, obligations of US govt agencies and CLOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	$ 12.5

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

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

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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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Amount, before tax, of unrealized loss 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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