Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Available-for-Sale:
Fair Value, Less than 12 Months	$ 2,118.0	$ 838.6
Gross Unrealized Losses, Less than 12 Months	(28.6)	(5.5)
Fair Value, 12 Months or Longer	0.0	16.6
Gross Unrealized Losses, 12 Months or Longer	0.0	(0.2)
Fair Value, Total	2,118.0	855.2
Gross Unrealized Losses, Total	(28.6)	(5.7)
Held-to-Maturity:
Fair Value, Less than 12 Months	771.3
Gross Unrealized Losses, Less than 12 Months	(9.1)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	771.3
Gross Unrealized Losses, Total	(9.1)
Debt Securities, Available-for-sale, Amortized Cost	3,949.4	3,932.7
Estimated Fair Value	3,957.8	4,008.7
Investment securities classified as held to maturity	1,685.5	51.6
State, county and municipal securities
Available-for-Sale:
Fair Value, Less than 12 Months	260.3	148.1
Gross Unrealized Losses, Less than 12 Months	(7.2)	(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	260.3	148.1
Gross Unrealized Losses, Total	(7.2)	(1.0)
Held-to-Maturity:
Debt Securities, Available-for-sale, Amortized Cost	449.2	462.1
Estimated Fair Value	444.5	465.9
Investment securities classified as held to maturity	72.4	46.6
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	293.7	235.6
Gross Unrealized Losses, Less than 12 Months	(6.7)	(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	293.7	235.6
Gross Unrealized Losses, Total	(6.7)	(2.0)
Held-to-Maturity:
Debt Securities, Available-for-sale, Amortized Cost	333.2	332.9
Estimated Fair Value	326.9	331.9
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Fair Value, Less than 12 Months	870.9	434.0
Gross Unrealized Losses, Less than 12 Months	(11.9)	(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	870.9	446.3
Gross Unrealized Losses, Total	(11.9)	(2.5)
Held-to-Maturity:
Fair Value, Less than 12 Months	742.0
Gross Unrealized Losses, Less than 12 Months	(8.6)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	742.0
Gross Unrealized Losses, Total	(8.6)
Debt Securities, Available-for-sale, Amortized Cost	2,064.7	2,830.8
Estimated Fair Value	2,082.2	2,897.6
Investment securities classified as held to maturity	1,591.5	1.0
Debt Securities, Unrealized Gain (Loss)	24.4
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)
Held-to-Maturity:
Debt Securities, Available-for-sale, Amortized Cost	16.9	10.9
Estimated Fair Value	17.0	10.9
Corporate securities
Available-for-Sale:
Fair Value, Less than 12 Months	196.2	20.9
Gross Unrealized Losses, Less than 12 Months	(2.6)	(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	196.2	20.9
Gross Unrealized Losses, Total	(2.6)	(0.1)
Held-to-Maturity:
Fair Value, Less than 12 Months	29.3
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	29.3
Gross Unrealized Losses, Total	(0.5)
Debt Securities, Available-for-sale, Amortized Cost	401.0	295.8
Estimated Fair Value	403.0	302.2
Investment securities classified as held to maturity	21.6	$ 3.9
Debt and Equity Securities, Unrealized Gain (Loss)	0.4
Residential Mortgage Backed Securities
Held-to-Maturity:
Debt Securities, Available-for-sale, Amortized Cost	291.6
Collateralized Loan Obligations
Held-to-Maturity:
Debt Securities, Available-for-sale, Amortized Cost	187.3
Estimated Fair Value	187.3
Available-for-sale Securities, Gross Unrealized Loss	0.0
Available-for-sale Securities, Gross Unrealized Gain	0.0
US Treasury Notes Securities
Available-for-Sale:
Fair Value, Less than 12 Months	496.9
Gross Unrealized Losses, Less than 12 Months	(0.2)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	496.9
Gross Unrealized Losses, Total	$ (0.2)