Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022	Jun. 30, 2023	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Available-for-Sale:
Fair Value, Less than 12 Months	$ 2.2	$ 5,071.9
Gross Unrealized Losses, Less than 12 Months	0.0	(346.6)
Fair Value, 12 Months or Longer	5,614.3	1,545.6
Gross Unrealized Losses, 12 Months or Longer	(662.6)	(283.1)
Fair Value, Total	5,616.5	6,617.5
Gross Unrealized Losses, Total	(662.6)	(629.7)
Allowance for Credit Losses	0.0	0.0	$ (1.4)	$ 0.0	$ 0.0	$ 0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	6,290.2	7,573.5
Gross Unrealized Gains	0.1	2.3
Gross Unrealized Losses	(662.6)	(629.7)
Estimated Fair Value	5,627.7	6,946.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	6,290.2	7,573.5
Investment securities classified as held to maturity	3,260.3	3,453.7
Allowance for Credit Losses	(0.8)	(1.9)	$ (0.7)	$ (1.9)	$ (1.6)	$ 0.0
Net Amortized Cost	3,259.5	3,451.8
Gross Unrealized Gains	0.0	0.4
Gross Unrealized Losses	(463.4)	(400.0)
Investment debt securities held-to-maturity	2,796.1	3,052.2
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	215.9
Gross Unrealized Losses, 12 Months or Longer	(34.2)
Fair Value, Total	215.9
Gross Unrealized Losses, Total	(34.2)
U.S. Treasury notes
Available-for-Sale:
Fair Value, Less than 12 Months		168.8
Gross Unrealized Losses, Less than 12 Months		(4.7)
Fair Value, 12 Months or Longer		170.4
Gross Unrealized Losses, 12 Months or Longer		(29.8)
Fair Value, Total		339.2
Gross Unrealized Losses, Total		(34.5)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	250.1	675.1
Gross Unrealized Gains	0.0	2.1
Gross Unrealized Losses	(34.2)	(34.5)
Estimated Fair Value	215.9	642.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	250.1	675.1
Investment securities classified as held to maturity	398.4	396.6
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	398.4	396.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(6.8)	(10.2)
Investment debt securities held-to-maturity	391.6	386.4
State, county, and municipal securities
Available-for-Sale:
Fair Value, Less than 12 Months	1.0	104.9
Gross Unrealized Losses, Less than 12 Months	0.0	(9.0)
Fair Value, 12 Months or Longer	199.7	146.1
Gross Unrealized Losses, 12 Months or Longer	(54.9)	(41.6)
Fair Value, Total	200.7	251.0
Gross Unrealized Losses, Total	(54.9)	(50.6)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	257.5	314.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(54.9)	(50.6)
Estimated Fair Value	202.6	263.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	257.5	314.3
Investment securities classified as held to maturity	179.0	181.2
Allowance for Credit Losses	0.0	(0.1)
Net Amortized Cost	179.0	181.1
Gross Unrealized Gains	0.0	0.2
Gross Unrealized Losses	(34.9)	(31.6)
Investment debt securities held-to-maturity	144.1	149.7
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	152.2
Gross Unrealized Losses, Less than 12 Months	0.0	(10.1)
Fair Value, 12 Months or Longer	161.5	46.1
Gross Unrealized Losses, 12 Months or Longer	(17.3)	(7.2)
Fair Value, Total	161.5	198.3
Gross Unrealized Losses, Total	(17.3)	(17.3)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	179.3	216.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(17.3)	(17.3)
Estimated Fair Value	162.0	198.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	179.3	216.2
Investment securities classified as held to maturity	353.8	351.7
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	353.8	351.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(59.5)	(49.6)
Investment debt securities held-to-maturity	294.3	302.1
Private mortgage-backed securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	133.1
Gross Unrealized Losses, Less than 12 Months	0.0	(19.1)
Fair Value, 12 Months or Longer	207.3	94.9
Gross Unrealized Losses, 12 Months or Longer	(39.1)	(17.8)
Fair Value, Total	207.3	228.0
Gross Unrealized Losses, Total	(39.1)	(36.9)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	246.4	264.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(39.1)	(36.9)
Estimated Fair Value	207.3	228.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	246.4	264.9
Collateralized Loan Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	1,082.6
Gross Unrealized Losses, Less than 12 Months	0.0	(33.1)
Fair Value, 12 Months or Longer	1,117.8	28.9
Gross Unrealized Losses, 12 Months or Longer	(8.9)	(0.5)
Fair Value, Total	1,117.8	1,111.5
Gross Unrealized Losses, Total	(8.9)	(33.6)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,126.7	1,145.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(8.9)	(33.6)
Estimated Fair Value	1,117.8	1,111.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,126.7	1,145.2
Corporate Debt Securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	130.6
Gross Unrealized Losses, Less than 12 Months	0.0	(8.3)
Fair Value, 12 Months or Longer	226.6	110.8
Gross Unrealized Losses, 12 Months or Longer	(34.6)	(22.5)
Fair Value, Total	226.6	241.4
Gross Unrealized Losses, Total	(34.6)	(30.8)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	261.1	272.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(34.6)	(30.8)
Estimated Fair Value	226.5	241.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	261.1	272.3
Investment securities classified as held to maturity	57.0	80.1
Allowance for Credit Losses	(0.8)	(1.8)
Net Amortized Cost	56.2	78.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(6.9)	(7.0)
Investment debt securities held-to-maturity	49.3	71.3
Commercial Mortgage-Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.3	1,069.8
Gross Unrealized Losses, Less than 12 Months	0.0	(59.9)
Fair Value, 12 Months or Longer	1,057.8	270.8
Gross Unrealized Losses, 12 Months or Longer	(125.1)	(57.0)
Fair Value, Total	1,058.1	1,340.6
Gross Unrealized Losses, Total	(125.1)	(116.9)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,191.9	1,468.0
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	(125.1)	(117.0)
Estimated Fair Value	1,066.9	1,351.2
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,191.9	1,468.0
Investment securities classified as held to maturity	515.2	526.8
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	515.2	526.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(71.9)	(58.3)
Investment debt securities held-to-maturity	443.3	468.5
Residential Mortgage Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.9	1,213.5
Gross Unrealized Losses, Less than 12 Months	0.0	(112.3)
Fair Value, 12 Months or Longer	1,284.0	344.6
Gross Unrealized Losses, 12 Months or Longer	(189.3)	(55.4)
Fair Value, Total	1,284.9	1,558.1
Gross Unrealized Losses, Total	(189.3)	(167.7)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,474.3	1,726.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(189.3)	(167.7)
Estimated Fair Value	1,285.0	1,558.3
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,474.3	1,726.0
Investment securities classified as held to maturity	1,269.8	1,391.5
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	1,269.8	1,391.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(197.2)	(166.6)
Investment debt securities held-to-maturity	1,072.6	1,224.9
Collateralized Mortgage Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	0.0	1,016.4
Gross Unrealized Losses, Less than 12 Months	0.0	(90.1)
Fair Value, 12 Months or Longer	1,143.7	333.0
Gross Unrealized Losses, 12 Months or Longer	(159.2)	(51.3)
Fair Value, Total	1,143.7	1,349.4
Gross Unrealized Losses, Total	(159.2)	(141.4)
Allowance for Credit Losses	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,302.9	1,491.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(159.2)	(141.3)
Estimated Fair Value	1,143.7	1,350.2
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,302.9	1,491.5
Investment securities classified as held to maturity	487.1	525.8
Allowance for Credit Losses	0.0	0.0
Net Amortized Cost	487.1	525.8
Gross Unrealized Gains	0.0	0.2
Gross Unrealized Losses	(86.2)	(76.7)
Investment debt securities held-to-maturity	400.9	449.3
US agency, residential and commercial MBS and CMOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	11.4	14.2
State, county & municipal, obligations of US govt agencies and CLOs
Available-for-Sale:
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	$ 18.6	$ 20.2