Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Jun. 07, 2021	Jun. 30, 2021	Dec. 31, 2020
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		$ 0.0	$ 0.0
Available-for-Sale:
Amortized Cost		3,949.4	3,932.7
Gross Unrealized Gains		37.0	81.7
Gross Unrealized Losses		(28.6)	(5.7)
Estimated Fair Value		3,957.8	4,008.7
Held-to-Maturity:
Amortized Cost		1,685.5	51.6
Gross Unrealized Gains		25.5	3.4
Gross Unrealized Losses		(9.1)	0.0
Estimated Fair Value		1,701.9	55.0
Debt Securities, Available-for-Sale and Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	$ 646.7
Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Allowance for Credit Loss, Expense	672.2
U.S. Treasury notes
Available-for-Sale:
Amortized Cost		497.1
Gross Unrealized Gains		0.0
Gross Unrealized Losses		(0.2)
Estimated Fair Value		496.9
State, county and municipal securities
Available-for-Sale:
Amortized Cost		449.2	462.1
Gross Unrealized Gains		2.5	4.8
Gross Unrealized Losses		(7.2)	(1.0)
Estimated Fair Value		444.5	465.9
Held-to-Maturity:
Amortized Cost		72.4	46.6
Gross Unrealized Gains		2.6	3.2
Gross Unrealized Losses		(0.5)	0.0
Estimated Fair Value		74.5	49.8
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost		333.2	332.9
Gross Unrealized Gains		0.4	1.0
Gross Unrealized Losses		(6.7)	(2.0)
Estimated Fair Value		326.9	331.9
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost		2,064.7	2,830.8
Gross Unrealized Gains		29.4	69.3
Gross Unrealized Losses		(11.9)	(2.5)
Estimated Fair Value		2,082.2	2,897.6
Held-to-Maturity:
Amortized Cost		1,591.5	1.0
Gross Unrealized Gains		22.4	0.1
Gross Unrealized Losses		(8.6)	0.0
Estimated Fair Value		1,605.3	1.1
Debt Securities, Available-for-Sale and Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	629.4
Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Allowance for Credit Loss, Expense	654.5
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost		16.9	10.9
Gross Unrealized Gains		0.1	0.1
Gross Unrealized Losses		0.0	(0.1)
Estimated Fair Value		17.0	10.9
Corporate securities
Available-for-Sale:
Amortized Cost		401.0	295.8
Gross Unrealized Gains		4.6	6.5
Gross Unrealized Losses		(2.6)	(0.1)
Estimated Fair Value		403.0	302.2
Held-to-Maturity:
Amortized Cost		21.6	3.9
Gross Unrealized Gains		0.5	0.1
Gross Unrealized Losses		0.0	0.0
Estimated Fair Value		$ 22.1	4.0
Debt Securities, Available-for-Sale and Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	17.3
Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Allowance for Credit Loss, Expense	$ 17.7
Other investments
Available-for-Sale:
Amortized Cost			0.2
Gross Unrealized Gains			0.0
Gross Unrealized Losses			0.0
Estimated Fair Value			0.2
Other Investments
Held-to-Maturity:
Amortized Cost			0.1
Gross Unrealized Gains			0.0
Gross Unrealized Losses			0.0
Estimated Fair Value			$ 0.1

Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]

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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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