Securities (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) investment	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) investment	Jun. 30, 2019 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss position | investment	447		447
Total fair value	$ 6,600,000,000		$ 6,600,000,000
Total unrealized losses	51,700,000		51,700,000
Provision for credit losses	66,200,000	$ (6,500,000)	127,000,000.0	$ (6,500,000)
Held to maturity securities, transferred from available for sale	301,500,000	301,500,000	301,500,000	301,500,000
Gross proceeds from sale of securities	$ 409,200,000	$ 101,800,000	$ 689,200,000	$ 229,300,000
A
Debt and Equity Securities, FV-NI [Line Items]
Percentage of debt securities, held-to-maturity, portfolio	0.28		0.28
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	$ 21,900,000		$ 21,900,000
Other Asset-Backed
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	24,400,000		24,400,000
Corporate Debt
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	$ 2,700,000		$ 2,700,000
Percent of corporate debt portfolio	22.00%		22.00%
Provision for credit losses	$ (200,000)		$ 0
U.S. Government
Debt and Equity Securities, FV-NI [Line Items]
Percentage of debt securities, held-to-maturity, portfolio	0.46		0.46
U.S. Government | AA
Debt and Equity Securities, FV-NI [Line Items]
Percentage of debt securities, held-to-maturity, portfolio	0.15		0.15
U.S. Government | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Percentage of debt securities, held-to-maturity, portfolio	0.11		0.11