Securities (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($) investment	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) investment	Jun. 30, 2021 USD ($)	Dec. 31, 2021 USD ($) investment
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss position | investment	2,484		2,484		1,233
Debt securities, unrealized losses	$ 1,835,700,000		$ 1,835,700,000		$ 248,600,000
Provision for credit losses	3,200,000	$ (27,000,000.0)	4,800,000	$ (57,000,000.0)
Net amount of recognized liabilities for repurchase agreements	799,400,000		799,400,000		531,900,000
Held to maturity securities	20,112,600,000		20,112,600,000		23,564,500,000
Debt securities, held-to-maturity, accumulated unrecognized loss	1,189,700,000		1,189,700,000		$ 283,600,000
Held to maturity securities, transferred from available for sale	$ 0	6,900,000,000	$ 0	6,900,000,000
Percentage of debt securities, held-to-maturity, portfolio	1		1		1
Realized investment gains (losses)	$ 0	0	$ 0	0
Gross proceeds from sale of securities	$ 0	0	$ 0	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	0.94		0.94		0.94
BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 317,100,000		$ 317,100,000		$ 345,000,000.0
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	$ 968,800,000		$ 968,800,000		$ 1,191,000,000
Percentage of debt securities, held-to-maturity, portfolio	0.05		0.05		0.05
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	$ 751,600,000		$ 751,600,000		$ 110,200,000
Held to maturity securities	5,806,700,000		5,806,700,000		5,927,600,000
Debt securities, held-to-maturity, accumulated unrecognized loss	582,500,000		582,500,000		106,100,000
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	430,000,000.0		430,000,000.0		38,600,000
Held to maturity securities					800,000
Debt securities, held-to-maturity, accumulated unrecognized loss					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	$ 63,500,000		$ 63,500,000		$ 9,700,000
Percent of corporate debt portfolio	12.00%		12.00%		14.00%
Provision for credit losses	$ 1,300,000	$ 0	$ 1,700,000	$ 0
Net amount of recognized liabilities for repurchase agreements	1,700,000		1,700,000		$ 0
Held to maturity securities	873,200,000		873,200,000		901,800,000
Debt securities, held-to-maturity, accumulated unrecognized loss	32,600,000		32,600,000		6,500,000
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	140,800,000		140,800,000		35,400,000
Held to maturity securities	6,065,800,000		6,065,800,000		6,098,000,000
Debt securities, held-to-maturity, accumulated unrecognized loss	273,600,000		273,600,000		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		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
Non-U.S. Government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	28,900,000		28,900,000		9,500,000
Held to maturity securities	3,097,000,000		3,097,000,000		5,773,300,000
Debt securities, held-to-maturity, accumulated unrecognized loss	79,500,000		79,500,000		9,800,000
Non-U.S. Government | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	316,000,000.0		316,000,000.0		343,900,000
Non-U.S. Government | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 0		$ 0		$ 0