Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2025	Dec. 31, 2024
Available-for-Sale:
Fair Value, Less than 12 Months	$ 408.0	$ 133.4
Gross Unrealized Losses, Less than 12 Months	(2.0)	(2.7)
Fair Value, 12 Months or Longer	3,997.0	4,047.2
Gross Unrealized Losses, 12 Months or Longer	(347.7)	(417.4)
Fair Value, Total	4,405.0	4,180.6
Gross Unrealized Losses, Total	(349.7)	(420.1)
Allowance for Credit Losses	0.0
Gross Unrealized Gains	2.7	2.4
Gross Unrealized Losses	(349.7)	(420.1)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	4,864.3	5,057.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	5,211.3	5,474.8
Investment securities classified as held to maturity	2,640.3	2,688.4
Allowance for Credit Losses	(0.8)	(0.9)
Net Amortized Cost	2,639.5	2,687.5
Gross Unrealized Gains	0.7	0.2
Gross Unrealized Losses	(279.8)	(330.0)
Investment debt securities held-to-maturity	2,361.2	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	231.2
Gross Unrealized Losses, 12 Months or Longer	(14.1)
Fair Value, Total	231.2
Gross Unrealized Losses, Total	(14.1)
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	(14.1)	(18.1)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	231.2	226.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	245.3	245.0
Investment securities classified as held to maturity	99.8	99.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.3)	(0.4)
Investment debt securities held-to-maturity	99.5	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	211.7	210.1
Gross Unrealized Losses, 12 Months or Longer	(37.8)	(40.0)
Fair Value, Total	211.7	210.1
Gross Unrealized Losses, Total	(37.8)	(40.0)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(37.8)	(40.0)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	213.7	212.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	251.5	252.0
Investment securities classified as held to maturity	176.2	176.3
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	(24.2)	(26.7)
Investment debt securities held-to-maturity	152.2	149.8
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	3.2	3.6
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	200.7	206.2
Gross Unrealized Losses, 12 Months or Longer	(7.9)	(10.8)
Fair Value, Total	203.9	209.8
Gross Unrealized Losses, Total	(7.9)	(10.8)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(7.9)	(10.8)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	207.5	213.3
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	215.4	224.1
Investment securities classified as held to maturity	468.2	470.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(50.0)	(59.9)
Investment debt securities held-to-maturity	418.2	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	187.5	190.4
Gross Unrealized Losses, 12 Months or Longer	(24.5)	(27.6)
Fair Value, Total	187.5	190.4
Gross Unrealized Losses, Total	(24.5)	(27.6)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(24.5)	(27.6)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	187.6	190.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	212.1	218.1
Collateralized Loan Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	336.8	10.0
Gross Unrealized Losses, Less than 12 Months	(1.0)	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	336.8	10.0
Gross Unrealized Losses, Total	(1.0)	0.0
Gross Unrealized Gains	0.2	1.3
Gross Unrealized Losses	(1.0)	0.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	633.9	772.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	634.7	770.7
Corporate Debt Securities
Available-for-Sale:
Fair Value, Less than 12 Months	27.8	27.5
Gross Unrealized Losses, Less than 12 Months	(0.2)	(0.5)
Fair Value, 12 Months or Longer	198.0	205.5
Gross Unrealized Losses, 12 Months or Longer	(13.3)	(16.2)
Fair Value, Total	225.8	233.0
Gross Unrealized Losses, Total	(13.5)	(16.7)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(13.5)	(16.7)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	225.8	232.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	239.3	249.6
Investment securities classified as held to maturity	56.9	57.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(1.9)	(2.8)
Investment debt securities held-to-maturity	55.0	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	938.3	934.9
Gross Unrealized Losses, 12 Months or Longer	(59.4)	(74.1)
Fair Value, Total	938.3	973.0
Gross Unrealized Losses, Total	(59.4)	(74.5)
Gross Unrealized Gains	0.5	0.1
Gross Unrealized Losses	(59.4)	(74.5)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	986.7	982.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,045.6	1,057.2
Investment securities classified as held to maturity	372.7	374.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(28.0)	(33.4)
Investment debt securities held-to-maturity	344.7	341.5
Residential Mortgage Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	33.3	33.7
Gross Unrealized Losses, Less than 12 Months	(0.7)	(1.5)
Fair Value, 12 Months or Longer	1,060.1	1,086.2
Gross Unrealized Losses, 12 Months or Longer	(100.9)	(124.3)
Fair Value, Total	1,093.4	1,119.9
Gross Unrealized Losses, Total	(101.6)	(125.8)
Gross Unrealized Gains	0.9	0.4
Gross Unrealized Losses	(101.6)	(125.8)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,149.3	1,175.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,250.0	1,301.2
Investment securities classified as held to maturity	1,049.2	1,082.5
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(117.0)	(140.7)
Investment debt securities held-to-maturity	932.3	941.8
Collateralized Mortgage Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	6.9	20.5
Gross Unrealized Losses, Less than 12 Months	(0.1)	(0.3)
Fair Value, 12 Months or Longer	969.5	987.1
Gross Unrealized Losses, 12 Months or Longer	(89.8)	(106.3)
Fair Value, Total	976.4	1,007.6
Gross Unrealized Losses, Total	(89.9)	(106.6)
Gross Unrealized Gains	1.1	0.6
Gross Unrealized Losses	(89.9)	(106.6)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,028.6	1,050.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,117.4	1,156.9
Investment securities classified as held to maturity	417.3	427.8
Gross Unrealized Gains	0.4	0.0
Gross Unrealized Losses	(58.4)	(66.1)
Investment debt securities held-to-maturity	359.3	$ 361.7
US agency, residential and commercial MBS and CMOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	(7.4)
State, county & municipal, obligations of US govt agencies and CLOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	$ 15.3

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

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