Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2024	Dec. 31, 2023
Available-for-Sale:
Fair Value, Less than 12 Months	$ 3.3	$ 0.9
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	4,295.7	5,505.5
Gross Unrealized Losses, 12 Months or Longer	(338.3)	(469.5)
Fair Value, Total	4,299.0	5,506.4
Gross Unrealized Losses, Total	(338.3)	(469.5)
Allowance for Credit Losses	0.0
Gross Unrealized Gains	6.6	3.2
Gross Unrealized Losses	(338.3)	(469.5)
Estimated Fair Value	5,534.3	5,841.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	5,866.0	6,307.8
Investment securities classified as held to maturity	2,742.0	3,208.7
Allowance for Credit Losses	(0.7)	(0.8)
Net Amortized Cost	2,741.3	3,207.9
Gross Unrealized Gains	1.6	0.4
Gross Unrealized Losses	(262.4)	(334.3)
Investment debt securities held-to-maturity	2,480.5	2,874.0
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss		18.1
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain		(10.7)
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	231.0
Gross Unrealized Losses, 12 Months or Longer	(19.2)
Fair Value, Total	231.0
Gross Unrealized Losses, Total	(19.2)
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		224.7
Gross Unrealized Losses, 12 Months or Longer		(25.5)
Fair Value, Total		224.7
Gross Unrealized Losses, Total		(25.5)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(19.2)	(25.5)
Estimated Fair Value	231.0	224.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	250.2	250.2
Investment securities classified as held to maturity	99.6	399.0
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	99.6	399.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.4)	(2.8)
Investment debt securities held-to-maturity	99.2	396.2
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	218.8	217.1
Gross Unrealized Losses, 12 Months or Longer	(31.9)	(36.9)
Fair Value, Total	218.8	217.1
Gross Unrealized Losses, Total	(31.9)	(36.9)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(31.9)	(36.9)
Estimated Fair Value	220.9	219.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	252.8	256.7
Investment securities classified as held to maturity	176.7	179.2
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	176.7	179.2
Gross Unrealized Gains	0.3	0.2
Gross Unrealized Losses	(20.6)	(24.2)
Investment debt securities held-to-maturity	156.4	155.2
Obligations of U.S. government agencies
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	165.1	168.0
Gross Unrealized Losses, 12 Months or Longer	(6.5)	(10.9)
Fair Value, Total	165.1	168.0
Gross Unrealized Losses, Total	(6.5)	(10.9)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(6.5)	(10.9)
Estimated Fair Value	165.4	168.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	171.9	179.4
Investment securities classified as held to maturity	356.4	354.5
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	356.4	354.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(32.0)	(42.4)
Investment debt securities held-to-maturity	324.4	312.1
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	199.5	210.5
Gross Unrealized Losses, 12 Months or Longer	(23.7)	(30.9)
Fair Value, Total	199.5	210.5
Gross Unrealized Losses, Total	(23.7)	(30.9)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(23.7)	(30.9)
Estimated Fair Value	199.6	210.4
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	223.3	241.3
Collateralized Loan Obligations
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		935.7
Gross Unrealized Losses, 12 Months or Longer		(2.3)
Fair Value, Total		935.7
Gross Unrealized Losses, Total		(2.3)
Gross Unrealized Gains	0.6	0.1
Gross Unrealized Losses	0.0	(2.3)
Estimated Fair Value	988.0	1,119.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	987.4	1,121.9
Corporate Debt 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	217.0	236.5
Gross Unrealized Losses, 12 Months or Longer	(14.9)	(24.4)
Fair Value, Total	217.0	236.5
Gross Unrealized Losses, Total	(14.9)	(24.4)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(14.9)	(24.4)
Estimated Fair Value	245.1	236.4
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	260.0	260.8
Investment securities classified as held to maturity	57.0	57.0
Allowance for Credit Losses	(0.7)	(0.8)
Net Amortized Cost	56.3	56.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(3.6)	(6.0)
Investment debt securities held-to-maturity	52.7	50.2
Commercial Mortgage-Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.1	0.2
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	1,048.8	1,083.1
Gross Unrealized Losses, 12 Months or Longer	(60.2)	(87.7)
Fair Value, Total	1,048.9	1,083.3
Gross Unrealized Losses, Total	(60.2)	(87.7)
Gross Unrealized Gains	1.4	0.6
Gross Unrealized Losses	(60.2)	(87.7)
Estimated Fair Value	1,105.3	1,105.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,164.1	1,192.7
Investment securities classified as held to maturity	493.0	510.5
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	493.0	510.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(41.8)	(52.9)
Investment debt securities held-to-maturity	451.2	457.6
Residential Mortgage Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	0.7
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	1,167.0	1,287.5
Gross Unrealized Losses, 12 Months or Longer	(94.2)	(130.6)
Fair Value, Total	1,167.0	1,288.2
Gross Unrealized Losses, Total	(94.2)	(130.6)
Gross Unrealized Gains	2.4	1.2
Gross Unrealized Losses	(94.2)	(130.6)
Estimated Fair Value	1,262.4	1,366.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,354.2	1,496.3
Investment securities classified as held to maturity	1,119.5	1,232.6
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	1,119.5	1,232.6
Gross Unrealized Gains	0.5	0.0
Gross Unrealized Losses	(106.2)	(137.0)
Investment debt securities held-to-maturity	1,013.8	1,095.6
Collateralized Mortgage Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	3.2	0.0
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	1,048.5	1,142.4
Gross Unrealized Losses, 12 Months or Longer	(87.7)	(120.3)
Fair Value, Total	1,051.7	1,142.4
Gross Unrealized Losses, Total	(87.7)	(120.3)
Gross Unrealized Gains	2.2	1.3
Gross Unrealized Losses	(87.7)	(120.3)
Estimated Fair Value	1,116.6	1,189.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,202.1	1,308.5
Investment securities classified as held to maturity	439.8	475.9
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	439.8	475.9
Gross Unrealized Gains	0.8	0.2
Gross Unrealized Losses	(57.8)	(69.0)
Investment debt securities held-to-maturity	382.8	$ 407.1
US agency, residential and commercial MBS and CMOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	(8.6)
State, county & municipal, obligations of US govt agencies and CLOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	$ 16.5

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 of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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