Securities (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024 USD ($) investment	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) investment
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss position | investment	784		898
Total fair value	$ 13,521.6		$ 19,435.3
Debt securities, unrealized losses	328.0		568.3
Investment Security Gains (Losses), net	189.4	$ (6.9)
Debt securities, available-for-sale, realized loss			176.4
Provision for (release of) credit losses	(7.8)	14.5
Held to maturity securities	23,497.9		26,221.7
Debt securities, held-to-maturity, accumulated unrecognized loss	$ 1,862.1		$ 1,755.6
Percentage of debt securities, held-to-maturity, portfolio	1		1
Moodys And Standard Poors A Or Higher Rating
Debt and Equity Securities, FV-NI [Line Items]
Percentage of debt securities, held-to-maturity, portfolio	0.96		0.97
BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 326.2		$ 334.0
Percentage of debt securities, held-to-maturity, portfolio	0.01		0.01
NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 576.6		$ 560.9
Percentage of debt securities, held-to-maturity, portfolio	0.03		0.02
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	$ 9,010.5		$ 9,817.6
Debt securities, unrealized losses	163.0		200.3
Investment Security Gains (Losses), net	23.0
Held to maturity securities	9,223.3		9,355.3
Debt securities, held-to-maturity, accumulated unrecognized loss	1,099.2		1,012.4
Government Sponsored Agency | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
Government Sponsored Agency | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
Obligations of States and Political Subdivisions
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	295.0		295.8
Debt securities, unrealized losses	14.6		20.0
Investment Security Gains (Losses), net		(9.8)
Held to maturity securities	2,571.6		2,563.9
Debt securities, held-to-maturity, accumulated unrecognized loss	89.5		72.4
Obligations of States and Political Subdivisions | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
Obligations of States and Political Subdivisions | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.2
Corporate Debt
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	92.6		147.4
Debt securities, unrealized losses	1.8		2.2
Investment Security Gains (Losses), net		1.2
Provision for (release of) credit losses	(0.7)	0.5
Net amount of recognized liabilities for repurchase agreements	0.5		1.2
Held to maturity securities	610.3		646.1
Debt securities, held-to-maturity, accumulated unrecognized loss	24.7		28.2
Proceeds from sale of investment securities	2,000.0	2,100.0
Corporate Debt | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
Corporate Debt | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
Commercial Mortgage-Backed
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	613.4		836.6
Debt securities, unrealized losses	18.0		48.4
Investment Security Gains (Losses), net	22.6	0.9
Held to maturity securities	37.6		37.6
Debt securities, held-to-maturity, accumulated unrecognized loss	0.3		0.8
Commercial Mortgage-Backed | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
Commercial Mortgage-Backed | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
Other Asset-Backed
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	1,008.9		2,018.5
Debt securities, unrealized losses	31.0		100.0
Investment Security Gains (Losses), net	56.6	$ 0.8
Held to maturity securities	193.7		214.2
Debt securities, held-to-maturity, accumulated unrecognized loss	0.2		0.2
Other Asset-Backed | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
Other Asset-Backed | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
U.S. Government
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	906.9		3,364.7
Debt securities, unrealized losses	20.3		61.5
Investment Security Gains (Losses), net	34.8
Held to maturity securities			0.0
U.S. Government | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities			0.0
U.S. Government | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities			0.0
Covered Bonds
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	165.3		213.2
Debt securities, unrealized losses	6.2		9.7
Investment Security Gains (Losses), net	4.2
Held to maturity securities	2,110.3		2,208.6
Debt securities, held-to-maturity, accumulated unrecognized loss	103.8		108.3
Covered Bonds | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
Covered Bonds | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	1,176.0		2,477.0
Debt securities, unrealized losses	55.6		105.8
Investment Security Gains (Losses), net	48.2
Held to maturity securities	5,024.5		5,245.5
Debt securities, held-to-maturity, accumulated unrecognized loss	289.4		294.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	1.1		1.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 0.0		$ 0.0