Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Available-for-Sale:
Fair Value, Less than 12 Months	$ 424.0	$ 0.9
Gross Unrealized Losses, Less than 12 Months	(1.8)	0.0
Fair Value, 12 Months or Longer	4,599.6	5,505.5
Gross Unrealized Losses, 12 Months or Longer	(493.5)	(469.5)
Fair Value, Total	5,023.6	5,506.4
Gross Unrealized Losses, Total	(495.3)	(469.5)
Allowance for Credit Losses	0.0	0.0	$ (2.6)	$ 0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	6,266.0	6,307.8
Gross Unrealized Gains	2.6	3.2
Gross Unrealized Losses	(495.3)	(469.5)
Estimated Fair Value	5,773.3	5,841.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	6,266.0	6,307.8
Investment securities classified as held to maturity	2,853.5	3,208.7
Allowance for Credit Losses	(0.7)	(0.8)	$ (0.7)	$ (1.9)
Net Amortized Cost	2,852.8	3,207.9
Gross Unrealized Gains	0.2	0.4
Gross Unrealized Losses	(355.2)	(334.3)
Investment debt securities held-to-maturity	2,497.8	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	222.4
Gross Unrealized Losses, 12 Months or Longer	(27.8)
Fair Value, Total	222.4
Gross Unrealized Losses, Total	(27.8)
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		224.7
Gross Unrealized Losses, 12 Months or Longer		(25.5)
Fair Value, Total		224.7
Gross Unrealized Losses, Total		(25.5)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	250.2	250.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(27.8)	(25.5)
Estimated Fair Value	222.4	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	99.4	399.0
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	99.4	399.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(1.7)	(2.8)
Investment debt securities held-to-maturity	97.7	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	213.4	217.1
Gross Unrealized Losses, 12 Months or Longer	(40.0)	(36.9)
Fair Value, Total	214.4	217.1
Gross Unrealized Losses, Total	(40.0)	(36.9)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	256.2	256.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(40.0)	(36.9)
Estimated Fair Value	216.2	219.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	256.2	256.7
Investment securities classified as held to maturity	179.0	179.2
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	179.0	179.2
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	(25.8)	(24.2)
Investment debt securities held-to-maturity	153.3	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	162.1	168.0
Gross Unrealized Losses, 12 Months or Longer	(11.9)	(10.9)
Fair Value, Total	162.1	168.0
Gross Unrealized Losses, Total	(11.9)	(10.9)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	174.4	179.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(11.9)	(10.9)
Estimated Fair Value	162.5	168.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	174.4	179.4
Investment securities classified as held to maturity	355.3	354.5
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	355.3	354.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(45.5)	(42.4)
Investment debt securities held-to-maturity	309.8	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	205.1	210.5
Gross Unrealized Losses, 12 Months or Longer	(31.2)	(30.9)
Fair Value, Total	205.1	210.5
Gross Unrealized Losses, Total	(31.2)	(30.9)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	236.2	241.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(31.2)	(30.9)
Estimated Fair Value	205.0	210.4
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	236.2	241.3
Collateralized Loan Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	317.6	0.0
Gross Unrealized Losses, Less than 12 Months	(0.3)	0.0
Fair Value, 12 Months or Longer	165.9	935.7
Gross Unrealized Losses, 12 Months or Longer	(0.2)	(2.3)
Fair Value, Total	483.5	935.7
Gross Unrealized Losses, Total	(0.5)	(2.3)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,116.2	1,121.9
Gross Unrealized Gains	0.7	0.1
Gross Unrealized Losses	(0.5)	(2.3)
Estimated Fair Value	1,116.4	1,119.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,116.2	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	237.0	236.5
Gross Unrealized Losses, 12 Months or Longer	(23.6)	(24.4)
Fair Value, Total	237.0	236.5
Gross Unrealized Losses, Total	(23.6)	(24.4)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	260.5	260.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(23.6)	(24.4)
Estimated Fair Value	236.9	236.4
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	260.5	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	(6.2)	(6.0)
Investment debt securities held-to-maturity	50.1	50.2
Commercial Mortgage-Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	19.1	0.2
Gross Unrealized Losses, Less than 12 Months	(0.2)	0.0
Fair Value, 12 Months or Longer	1,063.5	1,083.1
Gross Unrealized Losses, 12 Months or Longer	(93.9)	(87.7)
Fair Value, Total	1,082.6	1,083.3
Gross Unrealized Losses, Total	(94.1)	(87.7)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,213.6	1,192.7
Gross Unrealized Gains	0.3	0.6
Gross Unrealized Losses	(94.1)	(87.7)
Estimated Fair Value	1,119.8	1,105.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,213.6	1,192.7
Investment securities classified as held to maturity	500.2	510.5
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	500.2	510.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(57.9)	(52.9)
Investment debt securities held-to-maturity	442.3	457.6
Residential Mortgage Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	50.2	0.7
Gross Unrealized Losses, Less than 12 Months	(0.8)	0.0
Fair Value, 12 Months or Longer	1,231.1	1,287.5
Gross Unrealized Losses, 12 Months or Longer	(138.3)	(130.6)
Fair Value, Total	1,281.3	1,288.2
Gross Unrealized Losses, Total	(139.1)	(130.6)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,462.4	1,496.3
Gross Unrealized Gains	0.7	1.2
Gross Unrealized Losses	(139.1)	(130.6)
Estimated Fair Value	1,324.0	1,366.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,462.4	1,496.3
Investment securities classified as held to maturity	1,197.6	1,232.6
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	1,197.6	1,232.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(146.6)	(137.0)
Investment debt securities held-to-maturity	1,051.0	1,095.6
Collateralized Mortgage Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	36.1	0.0
Gross Unrealized Losses, Less than 12 Months	(0.5)	0.0
Fair Value, 12 Months or Longer	1,099.1	1,142.4
Gross Unrealized Losses, 12 Months or Longer	(126.6)	(120.3)
Fair Value, Total	1,135.2	1,142.4
Gross Unrealized Losses, Total	(127.1)	(120.3)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,296.3	1,308.5
Gross Unrealized Gains	0.9	1.3
Gross Unrealized Losses	(127.1)	(120.3)
Estimated Fair Value	1,170.1	1,189.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,296.3	1,308.5
Investment securities classified as held to maturity	465.0	475.9
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	465.0	475.9
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	(71.5)	(69.0)
Investment debt securities held-to-maturity	393.6	$ 407.1
US agency, residential and commercial MBS and CMOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	(10.0)
State, county & municipal, obligations of US govt agencies and CLOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	$ 17.6