Securities (Narrative) (Details) $ in Millions	3 Months Ended
Mar. 31, 2025 USD ($) security	Mar. 31, 2024 USD ($)	Dec. 31, 2024 USD ($) security
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss position \| security	730		767
Total fair value	$ 14,217.9		$ 17,832.2
Debt securities, unrealized losses	206.9		255.1
Investment security gains (losses)	0.0	$ (189.4)
Provision for (release of) credit losses	1.2	(7.8)
Held to maturity securities	20,874.3		22,296.7
Debt securities, held-to-maturity, accumulated unrecognized loss	$ 1,391.1		$ 1,645.1
Percentage of debt securities, held-to-maturity, portfolio	1		1
Proceeds from sale of investment securities	$ 0.0	2,000.0
Investment security loss	$ 0.0	189.4
Rating of A or higher
Debt and Equity Securities, FV-NI [Line Items]
Percentage of debt securities, held-to-maturity, portfolio	0.96		0.96
BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 282.6		$ 312.6
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	$ 619.1		$ 590.4
Percentage of debt securities, held-to-maturity, portfolio	0.03		0.03
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	$ 9,543.6		$ 9,672.6
Debt securities, unrealized losses	110.0		132.5
Held to maturity securities	8,485.6		8,635.0
Debt securities, held-to-maturity, accumulated unrecognized loss	910.6		1,081.3
Investment security loss	0.0	23.0
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
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	1,116.4		1,602.4
Debt securities, unrealized losses	34.7		42.2
Held to maturity securities	4,139.5		4,146.9
Debt securities, held-to-maturity, accumulated unrecognized loss	137.3		171.3
Investment security loss	0.0	48.2
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
U.S. Government
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	1,688.2		5,009.8
Debt securities, unrealized losses	10.9		22.9
Investment security loss	0.0	34.8
Other Asset-Backed
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	643.2		440.8
Debt securities, unrealized losses	9.6		12.9
Held to maturity securities	106.0		107.1
Debt securities, held-to-maturity, accumulated unrecognized loss	0.0		0.1
Investment security loss	0.0	56.6
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
Corporate Debt
Debt and Equity Securities, FV-NI [Line Items]
Total fair value	90.9		76.9
Debt securities, unrealized losses	2.0		1.6
Provision for (release of) credit losses	(0.2)	(0.7)
Net amount of recognized liabilities for repurchase agreements	0.0		0.2
Held to maturity securities	348.2		351.6
Debt securities, held-to-maturity, accumulated unrecognized loss	8.9		11.0
Investment security loss	0.0	$ 0.0
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
Other
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	625.7		621.7
Debt securities, held-to-maturity, accumulated unrecognized loss	175.9		172.5
Other \| BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0.0		0.0
Other \| NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 573.7		$ 571.0

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