Securities (Narrative) (Details)	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 31, 2023 USD ($)	Jun. 30, 2023 USD ($) investment	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($) investment	Jun. 30, 2022 USD ($)	Dec. 31, 2022 USD ($) investment
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss position | investment		1,045		1,045		1,030
Debt securities, unrealized losses		$ 914,100,000		$ 914,100,000		$ 1,049,200,000
Debt securities, available-for-sale, realized loss						213,000,000
Available for Sale		24,264,400,000		24,264,400,000		26,699,900,000
Debt securities, available-for-sale, realized gain	$ 6,900,000
Provision for credit losses		(15,000,000.0)	$ 3,200,000	(500,000)	$ 4,800,000
Held to maturity securities		26,006,300,000		26,006,300,000		25,036,100,000
Debt securities, held-to-maturity, accumulated unrecognized loss		$ 2,248,900,000		$ 2,248,900,000		$ 2,158,200,000
Percentage of debt securities, held-to-maturity, portfolio		1		1		1
Investment Security Gains (Losses), net		$ 0	0	$ 6,900,000	0
Moodys And Standard Poors A Or Higher Rating
Debt and Equity Securities, FV-NI [Line Items]
Percentage of debt securities, held-to-maturity, portfolio		0.97		0.97		0.97
BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		$ 330,200,000		$ 330,200,000		$ 323,400,000
Percentage of debt securities, held-to-maturity, portfolio		0.01		0.01		0.01
NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		$ 552,700,000		$ 552,700,000		$ 502,700,000
Percentage of debt securities, held-to-maturity, portfolio		0.02		0.02		0.02
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses		$ 311,300,000		$ 311,300,000		$ 351,600,000
Available for Sale		11,349,000,000		11,349,000,000		11,545,200,000
Held to maturity securities		9,354,200,000		9,354,200,000		9,407,700,000
Debt securities, held-to-maturity, accumulated unrecognized loss		1,137,200,000		1,137,200,000		1,076,000,000
Government Sponsored Agency | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		0		0		0
Government Sponsored Agency | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		0		0		0
Obligations of States and Political Subdivisions
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses		21,000,000.0		21,000,000.0		30,200,000
Available for Sale		289,200,000		289,200,000		787,600,000
Held to maturity securities		2,573,000,000		2,573,000,000		2,565,300,000
Debt securities, held-to-maturity, accumulated unrecognized loss		108,500,000		108,500,000		149,800,000
Obligations of States and Political Subdivisions | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		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
Corporate Debt
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses		10,700,000		10,700,000		17,400,000
Available for Sale		460,900,000		460,900,000		1,747,600,000
Provision for credit losses		(500,000)	1,300,000	0	1,700,000
Net amount of recognized liabilities for repurchase agreements		1,300,000		1,300,000		1,300,000
Held to maturity securities		669,900,000		669,900,000		713,300,000
Debt securities, held-to-maturity, accumulated unrecognized loss		46,700,000		46,700,000		45,400,000
Proceeds from sale of investment securities		0	0	2,100,000,000	$ 0
Investment Security Gains (Losses), net		0	$ 0
Corporate Debt | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		0		0		0
Corporate Debt | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		0		0		0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses		142,600,000		142,600,000		157,600,000
Available for Sale		2,918,000,000		2,918,000,000		2,479,400,000
Held to maturity securities		5,645,800,000		5,645,800,000		5,703,300,000
Debt securities, held-to-maturity, accumulated unrecognized loss		491,800,000		491,800,000		436,100,000
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		1,100,000		1,100,000		1,100,000
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
Available For Sale, Debt Securities, Portfolio For Sale
Debt and Equity Securities, FV-NI [Line Items]
Available for Sale						2,100,000,000
Commercial Mortgage-Backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses		64,200,000		64,200,000		69,200,000
Available for Sale		901,300,000		901,300,000		1,387,800,000
Held to maturity securities		37,600,000		37,600,000
Debt securities, held-to-maturity, accumulated unrecognized loss		800,000		800,000
Commercial Mortgage-Backed | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		0		0
Commercial Mortgage-Backed | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		0		0
Other Asset-Backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses		226,200,000		226,200,000		288,100,000
Available for Sale		4,909,600,000		4,909,600,000		5,256,200,000
Held to maturity securities		288,000,000.0		288,000,000.0		263,700,000
Debt securities, held-to-maturity, accumulated unrecognized loss		500,000		500,000		1,000,000.0
Other Asset-Backed | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		0		0		0
Other Asset-Backed | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities		$ 0		$ 0		$ 0