Securities (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) investment	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) investment	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss position | investment	738		738
Total fair value	$ 9,900,000,000		$ 9,900,000,000
Debt securities, unrealized losses	112,200,000		112,200,000		$ 31,300,000
Provision for credit losses	(27,000,000.0)	$ 66,200,000	(57,000,000.0)	$ 127,000,000.0
Held to maturity securities	21,655,300,000		21,655,300,000		17,791,100,000
Gross unrealized losses	145,800,000		145,800,000		$ 78,000,000.0
Held to maturity securities, transferred from available for sale	6,860,000,000	301,500,000	$ 6,860,000,000	301,500,000
Debt securities, held-to-maturity, unrealized gain (loss), transfer from available-for-sale	$ 120,100,000
Percentage of debt securities, held-to-maturity, portfolio	1		1		1
Gross proceeds from sale of securities	$ 0	409,200,000	$ 0	689,200,000
Debt securities, available-for-sale, realized gain		2,700,000		3,400,000
Debt securities, available-for-sale, realized loss		2,600,000		3,300,000
Realized investment gains (losses)	$ 0	100,000	$ 0	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	0.91		0.91		0.93
BBB
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities	$ 342,200,000		$ 342,200,000		$ 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	$ 1,372,000,000.0		$ 1,372,000,000.0		$ 1,261,800,000
Percentage of debt securities, held-to-maturity, portfolio	0.07		0.07		0.07
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	$ 64,700,000		$ 64,700,000		$ 26,900,000
Held to maturity securities	6,499,200,000		6,499,200,000		3,000,000.0
Gross unrealized losses	23,900,000		23,900,000		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
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	12,800,000		12,800,000		600,000
Held to maturity securities	5,333,100,000		5,333,100,000		3,648,000,000.0
Gross unrealized losses	19,000,000		19,000,000		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
Corporate Debt
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	$ 1,900,000		$ 1,900,000		100,000
Percent of corporate debt portfolio	16.00%		16.00%
Provision for credit losses	$ 0	(200,000)	$ 0
Debt Securities, available-for-sale, amortized cost, allowance for credit loss, excluding accrued interest	0	$ 0	0	$ 0	0
Held to maturity securities	880,900,000		880,900,000		588,000,000.0
Gross unrealized losses	1,000,000.0		1,000,000.0		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]
Debt securities, held-to-maturity, unrealized loss position, fair value	62,900,000		62,900,000
Obligations of States and Political Subdivisions
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	18,500,000		18,500,000		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