Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Available-for-Sale:
Fair Value, Less than 12 Months	$ 131.4	$ 0.9
Gross Unrealized Losses, Less than 12 Months	(2.3)	0.0
Fair Value, 12 Months or Longer	4,316.6	5,505.5
Gross Unrealized Losses, 12 Months or Longer	(491.1)	(469.5)
Fair Value, Total	4,448.0	5,506.4
Gross Unrealized Losses, Total	(493.4)	(469.5)
Allowance for Credit Losses	0.0	0.0	$ 0.0	$ (1.4)	$ (2.6)	$ 0.0
Gross Unrealized Gains	3.2	3.2
Gross Unrealized Losses	(493.4)	(469.5)
Estimated Fair Value	5,603.1	5,841.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	6,093.3	6,307.8
Investment securities classified as held to maturity	2,799.2	3,208.7
Allowance for Credit Losses	(0.7)	(0.8)	$ (0.7)	$ (0.7)	$ (0.7)	$ (1.9)
Net Amortized Cost	2,798.5	3,207.9
Gross Unrealized Gains	0.1	0.4
Gross Unrealized Losses	(358.2)	(334.3)
Investment debt securities held-to-maturity	2,440.4	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	222.9
Gross Unrealized Losses, 12 Months or Longer	(27.3)
Fair Value, Total	222.9
Gross Unrealized Losses, Total	(27.3)
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		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	(27.3)	(25.5)
Estimated Fair Value	222.9	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	99.5	399.0
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	99.5	399.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(1.6)	(2.8)
Investment debt securities held-to-maturity	97.9	396.2
State, county, and municipal securities
Available-for-Sale:
Fair Value, Less than 12 Months	1.0	0.0
Gross Unrealized Losses, Less than 12 Months	0.0	0.0
Fair Value, 12 Months or Longer	210.3	217.1
Gross Unrealized Losses, 12 Months or Longer	(40.9)	(36.9)
Fair Value, Total	211.3	217.1
Gross Unrealized Losses, Total	(40.9)	(36.9)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(40.9)	(36.9)
Estimated Fair Value	213.0	219.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	253.9	256.7
Investment securities classified as held to maturity	178.2	179.2
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	178.2	179.2
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	(26.7)	(24.2)
Investment debt securities held-to-maturity	151.6	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	162.6	168.0
Gross Unrealized Losses, 12 Months or Longer	(11.6)	(10.9)
Fair Value, Total	162.6	168.0
Gross Unrealized Losses, Total	(11.6)	(10.9)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(11.6)	(10.9)
Estimated Fair Value	162.9	168.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	174.5	179.4
Investment securities classified as held to maturity	356.0	354.5
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	356.0	354.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(44.9)	(42.4)
Investment debt securities held-to-maturity	311.1	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	198.4	210.5
Gross Unrealized Losses, 12 Months or Longer	(31.6)	(30.9)
Fair Value, Total	198.4	210.5
Gross Unrealized Losses, Total	(31.6)	(30.9)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(31.6)	(30.9)
Estimated Fair Value	198.4	210.4
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	230.0	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	1.8	0.1
Gross Unrealized Losses	0.0	(2.3)
Estimated Fair Value	1,068.6	1,119.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,066.8	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	237.9	236.5
Gross Unrealized Losses, 12 Months or Longer	(22.4)	(24.4)
Fair Value, Total	237.9	236.5
Gross Unrealized Losses, Total	(22.4)	(24.4)
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(22.4)	(24.4)
Estimated Fair Value	237.8	236.4
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	260.2	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	(6.4)	(6.0)
Investment debt securities held-to-maturity	49.9	50.2
Commercial Mortgage-Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	34.2	0.2
Gross Unrealized Losses, Less than 12 Months	(0.4)	0.0
Fair Value, 12 Months or Longer	1,052.1	1,083.1
Gross Unrealized Losses, 12 Months or Longer	(91.8)	(87.7)
Fair Value, Total	1,086.3	1,083.3
Gross Unrealized Losses, Total	(92.2)	(87.7)
Gross Unrealized Gains	0.3	0.6
Gross Unrealized Losses	(92.2)	(87.7)
Estimated Fair Value	1,107.7	1,105.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,199.6	1,192.7
Investment securities classified as held to maturity	496.2	510.5
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	496.2	510.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(57.9)	(52.9)
Investment debt securities held-to-maturity	438.3	457.6
Residential Mortgage Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	61.6	0.7
Gross Unrealized Losses, Less than 12 Months	(1.3)	0.0
Fair Value, 12 Months or Longer	1,176.3	1,287.5
Gross Unrealized Losses, 12 Months or Longer	(140.0)	(130.6)
Fair Value, Total	1,237.9	1,288.2
Gross Unrealized Losses, Total	(141.3)	(130.6)
Gross Unrealized Gains	0.4	1.2
Gross Unrealized Losses	(141.3)	(130.6)
Estimated Fair Value	1,267.2	1,366.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,408.1	1,496.3
Investment securities classified as held to maturity	1,159.6	1,232.6
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	1,159.6	1,232.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(148.7)	(137.0)
Investment debt securities held-to-maturity	1,010.9	1,095.6
Collateralized Mortgage Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	34.6	0.0
Gross Unrealized Losses, Less than 12 Months	(0.6)	0.0
Fair Value, 12 Months or Longer	1,056.1	1,142.4
Gross Unrealized Losses, 12 Months or Longer	(125.5)	(120.3)
Fair Value, Total	1,090.7	1,142.4
Gross Unrealized Losses, Total	(126.1)	(120.3)
Gross Unrealized Gains	0.7	1.3
Gross Unrealized Losses	(126.1)	(120.3)
Estimated Fair Value	1,124.6	1,189.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,250.0	1,308.5
Investment securities classified as held to maturity	452.7	475.9
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	452.7	475.9
Gross Unrealized Gains	0.0	0.2
Gross Unrealized Losses	(72.0)	(69.0)
Investment debt securities held-to-maturity	380.7	$ 407.1
US agency, residential and commercial MBS and CMOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	(9.3)
State, county & municipal, obligations of US govt agencies and CLOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	$ 17.0