Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Available-for-Sale:
Fair Value, Less than 12 Months	$ 3,860.7	$ 838.6
Gross Unrealized Losses, Less than 12 Months	(53.1)	(5.5)
Fair Value, 12 Months or Longer	11.3	16.6
Gross Unrealized Losses, 12 Months or Longer	(0.1)	(0.2)
Fair Value, Total	3,872.0	855.2
Gross Unrealized Losses, Total	(53.2)	(5.7)
Held-to-Maturity:
Fair Value, Less than 12 Months	851.5
Gross Unrealized Losses, Less than 12 Months	(13.7)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	851.5
Gross Unrealized Losses, Total	(13.7)
State, county and municipal securities
Available-for-Sale:
Fair Value, Less than 12 Months	444.8	148.1
Gross Unrealized Losses, Less than 12 Months	(14.7)	(1.0)
Fair Value, 12 Months or Longer	0.0	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0	0.0
Fair Value, Total	444.8	148.1
Gross Unrealized Losses, Total	(14.7)	(1.0)
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	323.6	235.6
Gross Unrealized Losses, Less than 12 Months	(11.8)	(2.0)
Fair Value, 12 Months or Longer	0.0	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0	0.0
Fair Value, Total	323.6	235.6
Gross Unrealized Losses, Total	(11.8)	(2.0)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Fair Value, Less than 12 Months	2,750.0	434.0
Gross Unrealized Losses, Less than 12 Months	(20.7)	(2.4)
Fair Value, 12 Months or Longer	11.3	12.3
Gross Unrealized Losses, 12 Months or Longer	(0.1)	(0.1)
Fair Value, Total	2,761.3	446.3
Gross Unrealized Losses, Total	(20.8)	(2.5)
Held-to-Maturity:
Fair Value, Less than 12 Months	775.6
Gross Unrealized Losses, Less than 12 Months	(12.8)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	775.6
Gross Unrealized Losses, Total	(12.8)
Private mortgage-backed 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		4.3
Gross Unrealized Losses, 12 Months or Longer		(0.1)
Fair Value, Total		4.3
Gross Unrealized Losses, Total		(0.1)
Corporate securities
Available-for-Sale:
Fair Value, Less than 12 Months	342.3	20.9
Gross Unrealized Losses, Less than 12 Months	(5.9)	(0.1)
Fair Value, 12 Months or Longer	0.0	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0	0.0
Fair Value, Total	342.3	20.9
Gross Unrealized Losses, Total	(5.9)	$ (0.1)
Held-to-Maturity:
Fair Value, Less than 12 Months	75.9
Gross Unrealized Losses, Less than 12 Months	(0.9)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	75.9
Gross Unrealized Losses, Total	$ (0.9)

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