Securities (Narrative) (Details)	3 Months Ended
	Mar. 31, 2022 USD ($) investment	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($) investment
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss position | investment	2,196		1,233
Debt securities, unrealized losses	$ 1,157,700,000		$ 248,600,000
Provision for credit losses	1,600,000	$ (30,000,000.0)
Net Amount of Recognized Liabilities for Repurchase Agreements in Note 23	329,700,000		531,900,000
Held to maturity securities	21,223,500,000		23,564,500,000
GROSS UNREALIZED LOSSES	835,200,000		$ 283,600,000
Held to maturity securities, transferred from available for sale	$ 0	0
Percentage of debt securities, held-to-maturity, portfolio	1		1
Realized investment gains (losses)	$ 0	0
Gross proceeds from sale of securities		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	0.92		0.94
BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 337,400,000		$ 345,000,000.0
Percentage of debt securities, held-to-maturity, portfolio	0.02		0.01
NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 1,361,300,000		$ 1,191,000,000
Percentage of debt securities, held-to-maturity, portfolio	0.06		0.05
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	$ 475,900,000		$ 110,200,000
Held to maturity securities	6,079,800,000		5,927,600,000
GROSS UNREALIZED LOSSES	400,900,000		106,100,000
Government Sponsored Agency | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0		0
Government Sponsored Agency | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0		0
Obligations of States and Political Subdivisions
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	256,600,000		38,600,000
Held to maturity securities			800,000
GROSS UNREALIZED LOSSES			0
Obligations of States and Political Subdivisions | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities			0
Obligations of States and Political Subdivisions | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities			0
Corporate Debt
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	$ 45,200,000		$ 9,700,000
Percent of corporate debt portfolio	12.00%		14.00%
Provision for credit losses	$ 400,000	$ 0
Net Amount of Recognized Liabilities for Repurchase Agreements in Note 23	400,000		$ 0
Held to maturity securities	932,200,000		901,800,000
GROSS UNREALIZED LOSSES	19,000,000.0		6,500,000
Corporate Debt | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0		0
Corporate Debt | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0		0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	114,000,000.0		35,400,000
Held to maturity securities	6,223,600,000		6,098,000,000
GROSS UNREALIZED LOSSES	208,200,000		80,000,000
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
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
Non-U.S. Government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	24,400,000		9,500,000
Held to maturity securities	2,950,900,000		5,773,300,000
GROSS UNREALIZED LOSSES	50,600,000		9,800,000
Non-U.S. Government | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	336,300,000		343,900,000
Non-U.S. Government | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 0		$ 0