Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022	Dec. 31, 2021
Available-for-Sale:
Gross Unrealized Gains	$ 0.2		$ 2.3
Gross Unrealized Losses	(533.1)		(629.7)
Held-to-Maturity:
Investment securities classified as held to maturity	3,376.9		3,453.7
Allowance for Credit Losses	(0.7)	$ (1.6)	(1.9)	$ 0.0
Net Amortized Cost	3,376.2		3,451.8
Gross Unrealized Gains	1.4		0.4
Gross Unrealized Losses	(351.8)		(400.0)
Investment debt securities held-to-maturity	3,025.8		3,052.2
Estimated Fair Value	6,049.3		6,946.1
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	6,582.2		7,573.5
U.S. Treasury notes
Available-for-Sale:
Gross Unrealized Gains	0.0		2.1
Gross Unrealized Losses	(26.9)		(34.5)
Held-to-Maturity:
Investment securities classified as held to maturity	397.2		396.6
Allowance for Credit Losses	0.0		0.0
Net Amortized Cost	397.2		396.6
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(7.6)		(10.2)
Investment debt securities held-to-maturity	389.6		386.4
Estimated Fair Value	223.2		642.7
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	250.1		675.1
State, county, and municipal securities
Available-for-Sale:
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(40.9)		(50.6)
Held-to-Maturity:
Investment securities classified as held to maturity	180.9		181.2
Allowance for Credit Losses	0.0		(0.1)
Net Amortized Cost	180.9		181.1
Gross Unrealized Gains	0.4		0.2
Gross Unrealized Losses	(25.2)		(31.6)
Investment debt securities held-to-maturity	156.1		149.7
Estimated Fair Value	221.1		263.7
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	262.0		314.3
Obligations of U.S. government agencies
Available-for-Sale:
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(12.3)		(17.3)
Held-to-Maturity:
Investment securities classified as held to maturity	352.3		351.7
Allowance for Credit Losses	0.0		0.0
Net Amortized Cost	352.3		351.7
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(44.7)		(49.6)
Investment debt securities held-to-maturity	307.6		302.1
Estimated Fair Value	174.1		198.9
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	186.4		216.2
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Gross Unrealized Gains	0.2		0.2
Gross Unrealized Losses	(362.8)		(426.0)
Held-to-Maturity:
Investment securities classified as held to maturity	2,389.4		2,444.1
Allowance for Credit Losses	0.0		0.0
Net Amortized Cost	2,389.4		2,444.1
Gross Unrealized Gains	1.0		0.2
Gross Unrealized Losses	(269.7)		(301.6)
Investment debt securities held-to-maturity	2,120.7		2,142.7
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	13.2	$ 20.1
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	(19.7)
Estimated Fair Value	3,858.4		4,259.7
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	4,221.0		4,685.5
Private mortgage-backed securities
Available-for-Sale:
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(35.2)		(36.9)
Held-to-Maturity:
Estimated Fair Value	225.1		228.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	260.3		264.9
Collateralized Loan Obligations
Available-for-Sale:
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(29.4)		(33.6)
Held-to-Maturity:
Estimated Fair Value	1,113.9		1,111.6
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,143.3		1,145.2
Corporate Debt Securities
Available-for-Sale:
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(25.6)		(30.8)
Held-to-Maturity:
Investment securities classified as held to maturity	57.1		80.1
Allowance for Credit Losses	(0.7)		(1.8)
Net Amortized Cost	56.4		78.3
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(4.6)		(7.0)
Investment debt securities held-to-maturity	51.8		71.3
Estimated Fair Value	233.5		241.5
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	$ 259.1		$ 272.3

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