Securities (Narrative) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) investment	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) investment	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss position | investment	966		966
Total fair value	$ 11,900,000,000		$ 11,900,000,000
Debt securities, unrealized losses	152,800,000		152,800,000		$ 31,300,000
Provision for credit losses	(13,000,000.0)	$ 500,000	(70,000,000.0)	$ 127,500,000
Held to maturity securities	20,797,800,000		20,797,800,000		17,791,100,000
Gross unrealized losses	159,900,000		159,900,000		$ 78,000,000.0
Held to maturity securities, transferred from available for sale	$ 0	0	$ 6,900,000,000	301,500,000
Percentage of debt securities, held-to-maturity, portfolio	1		1		1
Gross proceeds from sale of securities		0		689,200,000
Debt securities, available-for-sale, realized gain				3,400,000
Debt securities, available-for-sale, realized loss				3,300,000
Realized investment gains (losses)	$ 100,000	0	$ 100,000	100,000
Moodys And Standard Poors A Or Higher Rating
Debt and Equity Securities, FV-NI [Line Items]
Percentage of debt securities, held-to-maturity, portfolio	0.93		0.93		0.93
BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 334,300,000		$ 334,300,000		$ 49,900,000
Percentage of debt securities, held-to-maturity, portfolio	0.02		0.02		0
NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 1,250,700,000		$ 1,250,700,000		$ 1,261,800,000
Percentage of debt securities, held-to-maturity, portfolio	0.05		0.05		0.07
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	$ 90,900,000		$ 90,900,000		$ 26,900,000
Held to maturity securities	6,208,800,000		6,208,800,000		3,000,000.0
Gross unrealized losses	47,400,000		47,400,000		0
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	27,300,000		27,300,000		100,000
Held to maturity securities	1,000,000.0		1,000,000.0		2,100,000
Gross unrealized losses	0		0		0
Obligations of States and Political Subdivisions | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	0		0		1,100,000
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	$ 2,400,000		$ 2,400,000		100,000
Percent of corporate debt portfolio	15.00%		15.00%
Provision for credit losses	$ 0	$ 0	$ 0	$ 0
Debt Securities, available-for-sale, amortized cost, allowance for credit loss, excluding accrued interest	0		0		0
Held to maturity securities	945,800,000		945,800,000		588,000,000.0
Gross unrealized losses	2,300,000		2,300,000		100,000
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
Other Residential Mortgage-Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Gross unrealized losses	61,100,000		61,100,000
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	14,100,000		14,100,000		600,000
Held to maturity securities	5,804,900,000		5,804,900,000		3,648,000,000
Gross unrealized losses	41,900,000		41,900,000		900,000
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	1,200,000		1,200,000		0
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	4,000,000.0		4,000,000.0		700,000
Held to maturity securities	2,792,800,000		2,792,800,000		8,336,600,000
Gross unrealized losses	3,700,000		3,700,000		200,000
Non-U.S. Government | BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	333,100,000		333,100,000		48,800,000
Non-U.S. Government | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 0		$ 0		$ 0