Securities (Narrative) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) investment	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) investment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss position | investment	475		475
Total fair value	$ 7,700,000,000		$ 7,700,000,000
Total unrealized losses	35,000,000.0		35,000,000.0
Provision for credit losses	500,000	$ (7,000,000.0)	127,500,000	$ (13,500,000)
Debt securities, held-to-maturity	16,754,000,000.0		16,754,000,000.0		$ 12,284,500,000
Debt Securities, held-to-maturity, unrealized loss position, fair value	88,700,000		88,700,000
Held to maturity securities, transferred from available for sale	0	160,800,000	301,500,000	160,800,000
Gross proceeds from sale of securities	0	$ 177,300,000	689,200,000	$ 406,600,000
AA
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	2,491,900,000		2,491,900,000
A
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	$ 6,172,500,000		$ 6,172,500,000
Percentage of debt securities, held-to-maturity, portfolio	0.37		0.37
NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	$ 1,168,900,000		$ 1,168,900,000
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	21,200,000		21,200,000
Debt securities, held-to-maturity	3,400,000		3,400,000		4,100,000
Government Sponsored Agency | AA
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	0		0
Government Sponsored Agency | A
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	0		0
Government Sponsored Agency | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	0		0
Other Asset-Backed
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	9,500,000		9,500,000
Debt securities, held-to-maturity	645,800,000		645,800,000		804,300,000
Other Asset-Backed | AA
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	0		0
Other Asset-Backed | A
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	0		0
Other Asset-Backed | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	0		0
Corporate Debt
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	$ 2,200,000		$ 2,200,000
Percent of corporate debt portfolio	21.00%		21.00%
Provision for credit losses	$ 0		$ 0
Debt securities, held-to-maturity	530,300,000		530,300,000		405,100,000
Corporate Debt | AA
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	254,700,000		254,700,000
Corporate Debt | A
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	272,000,000.0		272,000,000.0
Corporate Debt | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	0		0
Other
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	442,600,000		442,600,000		291,100,000
Other | AA
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	0		0
Other | A
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	0		0
Other | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	442,600,000		442,600,000
U.S. Government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	$ 99,000,000.0		$ 99,000,000.0		$ 138,800,000
Percentage of debt securities, held-to-maturity, portfolio	0.41		0.41
U.S. Government | AA
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	$ 0		$ 0
Percentage of debt securities, held-to-maturity, portfolio	0.15		0.15
U.S. Government | A
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	$ 0		$ 0
U.S. Government | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	$ 0		$ 0
Percentage of debt securities, held-to-maturity, portfolio	0.07		0.07