Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Available-for-Sale:
Fair Value, Less than 12 Months	$ 1,728.8	$ 5,071.9
Gross Unrealized Losses, Less than 12 Months	(74.9)	(346.6)
Fair Value, 12 Months or Longer	4,306.5	1,545.6
Gross Unrealized Losses, 12 Months or Longer	(458.2)	(283.1)
Fair Value, Total	6,035.3	6,617.5
Gross Unrealized Losses, Total	(533.1)	(629.7)
Allowance for Credit Losses	(2.6)	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	6,582.2	7,573.5
Gross Unrealized Gains	0.2	2.3
Gross Unrealized Losses	(533.1)	(629.7)
Estimated Fair Value	6,049.3	6,946.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	6,584.8
US Treasury Notes Securities
Available-for-Sale:
Fair Value, Less than 12 Months	48.1
Gross Unrealized Losses, Less than 12 Months	(1.3)
Fair Value, 12 Months or Longer	175.1
Gross Unrealized Losses, 12 Months or Longer	(25.6)
Fair Value, Total	223.2
Gross Unrealized Losses, Total	(26.9)
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
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	(26.9)	(34.5)
Estimated Fair Value	223.2	642.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	250.1
State, county, and municipal securities
Available-for-Sale:
Fair Value, Less than 12 Months	8.0	104.9
Gross Unrealized Losses, Less than 12 Months	(0.3)	(9.0)
Fair Value, 12 Months or Longer	210.0	146.1
Gross Unrealized Losses, 12 Months or Longer	(40.6)	(41.6)
Fair Value, Total	218.0	251.0
Gross Unrealized Losses, Total	(40.9)	(50.6)
Allowance for Credit Losses	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	262.0	314.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(40.9)	(50.6)
Estimated Fair Value	221.1	263.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	262.0
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	86.7	152.2
Gross Unrealized Losses, Less than 12 Months	(4.0)	(10.1)
Fair Value, 12 Months or Longer	86.8	46.1
Gross Unrealized Losses, 12 Months or Longer	(8.3)	(7.2)
Fair Value, Total	173.5	198.3
Gross Unrealized Losses, Total	(12.3)	(17.3)
Allowance for Credit Losses	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	186.4	216.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(12.3)	(17.3)
Estimated Fair Value	174.1	198.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	186.4
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Fair Value, Less than 12 Months	1,388.6	3,299.7
Gross Unrealized Losses, Less than 12 Months	(63.5)	(262.3)
Fair Value, 12 Months or Longer	2,459.5	948.4
Gross Unrealized Losses, 12 Months or Longer	(299.3)	(163.7)
Fair Value, Total	3,848.1	4,248.1
Gross Unrealized Losses, Total	(362.8)	(426.0)
Allowance for Credit Losses	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	4,221.0	4,685.5
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	(362.8)	(426.0)
Estimated Fair Value	3,858.4	4,259.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	4,221.0
Private mortgage-backed securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.1	133.1
Gross Unrealized Losses, Less than 12 Months	0.0	(19.1)
Fair Value, 12 Months or Longer	225.0	94.9
Gross Unrealized Losses, 12 Months or Longer	(35.2)	(17.8)
Fair Value, Total	225.1	228.0
Gross Unrealized Losses, Total	(35.2)	(36.9)
Allowance for Credit Losses	0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	260.3	264.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(35.2)	(36.9)
Estimated Fair Value	225.1	228.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	260.3
Collateralized Loan Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	117.3	1,082.6
Gross Unrealized Losses, Less than 12 Months	(3.0)	(33.1)
Fair Value, 12 Months or Longer	996.6	28.9
Gross Unrealized Losses, 12 Months or Longer	(26.4)	(0.5)
Fair Value, Total	1,113.9	1,111.5
Gross Unrealized Losses, Total	(29.4)	(33.6)
Allowance for Credit Losses
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,143.3	1,145.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(29.4)	(33.6)
Estimated Fair Value	1,113.9	1,111.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,143.3
Corporate Debt Securities
Available-for-Sale:
Fair Value, Less than 12 Months	80.0	130.6
Gross Unrealized Losses, Less than 12 Months	(2.8)	(8.3)
Fair Value, 12 Months or Longer	153.5	110.8
Gross Unrealized Losses, 12 Months or Longer	(22.8)	(22.5)
Fair Value, Total	233.5	241.4
Gross Unrealized Losses, Total	(25.6)	(30.8)
Allowance for Credit Losses	(2.6)
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	259.1	272.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(25.6)	(30.8)
Estimated Fair Value	233.5	$ 241.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	$ 261.7

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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