Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Available-for-Sale:
Fair Value, Less than 12 Months	$ 2,458.6	$ 838.6
Gross Unrealized Losses, Less than 12 Months	(38.2)	(5.5)
Fair Value, 12 Months or Longer	21.2	16.6
Gross Unrealized Losses, 12 Months or Longer	(0.1)	(0.2)
Fair Value, Total	2,479.8	855.2
Gross Unrealized Losses, Total	(38.3)	(5.7)
Held-to-Maturity:
Fair Value, Less than 12 Months	801.3
Gross Unrealized Losses, Less than 12 Months	(8.9)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	801.3
Gross Unrealized Losses, Total	(8.9)
Debt Securities, Available-for-sale, Amortized Cost	4,408.8	3,932.7
Estimated Fair Value	4,403.6	4,008.7
Available-for-sale Securities, Gross Unrealized Loss	38.3	5.7
Gross Unrealized Gains	33.1	81.7
Investment securities classified as held to maturity	1,618.1	51.6
State, county and municipal securities
Available-for-Sale:
Fair Value, Less than 12 Months	287.7	148.1
Gross Unrealized Losses, Less than 12 Months	(7.1)	(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	287.7	148.1
Gross Unrealized Losses, Total	(7.1)	(1.0)
Held-to-Maturity:
Debt Securities, Available-for-sale, Amortized Cost	444.9	462.1
Estimated Fair Value	439.9	465.9
Available-for-sale Securities, Gross Unrealized Loss	7.1	1.0
Gross Unrealized Gains	2.1	4.8
Investment securities classified as held to maturity	69.7	46.6
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	301.2	235.6
Gross Unrealized Losses, Less than 12 Months	(6.6)	(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	301.2	235.6
Gross Unrealized Losses, Total	(6.6)	(2.0)
Held-to-Maturity:
Debt Securities, Available-for-sale, Amortized Cost	332.3	332.9
Estimated Fair Value	325.9	331.9
Available-for-sale Securities, Gross Unrealized Loss	6.6	2.0
Gross Unrealized Gains	0.2	1.0
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Fair Value, Less than 12 Months	964.9	434.0
Gross Unrealized Losses, Less than 12 Months	(16.0)	(2.4)
Fair Value, 12 Months or Longer	0.0	12.3
Gross Unrealized Losses, 12 Months or Longer	0.0	(0.1)
Fair Value, Total	964.9	446.3
Gross Unrealized Losses, Total	(16.0)	(2.5)
Held-to-Maturity:
Fair Value, Less than 12 Months	772.3
Gross Unrealized Losses, Less than 12 Months	(8.4)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	772.3
Gross Unrealized Losses, Total	(8.4)
Debt Securities, Available-for-sale, Amortized Cost	2,021.4	2,830.8
Estimated Fair Value	2,031.3	2,897.6
Available-for-sale Securities, Gross Unrealized Loss	16.0	2.5
Gross Unrealized Gains	25.9	69.3
Investment securities classified as held to maturity	1,537.5	1.0
Debt Securities, Unrealized Gain (Loss)	22.3
Private mortgage-backed securities
Available-for-Sale:
Fair Value, Less than 12 Months	54.3	0.0
Gross Unrealized Losses, Less than 12 Months	(0.3)	0.0
Fair Value, 12 Months or Longer	0.0	4.3
Gross Unrealized Losses, 12 Months or Longer		(0.1)
Fair Value, Total	54.3	4.3
Gross Unrealized Losses, Total	(0.3)	(0.1)
Held-to-Maturity:
Debt Securities, Available-for-sale, Amortized Cost	83.7	10.9
Estimated Fair Value	83.5	10.9
Available-for-sale Securities, Gross Unrealized Loss	0.3	0.1
Gross Unrealized Gains	0.1	0.1
Corporate securities
Available-for-Sale:
Fair Value, Less than 12 Months	158.7	20.9
Gross Unrealized Losses, Less than 12 Months	(2.6)	(0.1)
Fair Value, 12 Months or Longer	21.2	0.0
Gross Unrealized Losses, 12 Months or Longer	(0.1)	0.0
Fair Value, Total	179.9	20.9
Gross Unrealized Losses, Total	(2.7)	(0.1)
Held-to-Maturity:
Fair Value, Less than 12 Months	29.0
Gross Unrealized Losses, Less than 12 Months	(0.5)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	29.0
Gross Unrealized Losses, Total	(0.5)
Debt Securities, Available-for-sale, Amortized Cost	376.7	295.8
Estimated Fair Value	378.2	302.2
Available-for-sale Securities, Gross Unrealized Loss	2.7	0.1
Gross Unrealized Gains	4.2	6.5
Investment securities classified as held to maturity	10.9	$ 3.9
Residential Mortgage Backed Securities
Held-to-Maturity:
Debt Securities, Available-for-sale, Amortized Cost	540.2
Collateralized Loan Obligations
Held-to-Maturity:
Debt Securities, Available-for-sale, Amortized Cost	452.4
Estimated Fair Value	453.0
Available-for-sale Securities, Gross Unrealized Loss	0.0
Gross Unrealized Gains	0.6
US Treasury Notes Securities
Available-for-Sale:
Fair Value, Less than 12 Months	691.8
Gross Unrealized Losses, Less than 12 Months	(5.6)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	691.8
Gross Unrealized Losses, Total	$ (5.6)