Securities (Narrative) (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) investment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss position | investment	412
Total fair value	$ 6,400,000,000
Total unrealized losses	31,300,000
Provision for credit losses	125,000,000.0	$ (14,500,000)	$ (14,500,000)
Held to Maturity	17,791,100,000	12,284,500,000
Debt Securities, held-to-maturity, unrealized loss position, fair value	76,500,000
Transferred from available for sale to held to maturity	$ 301,500,000	160,800,000
Percentage of debt securities, held-to-maturity, portfolio	1
Gross proceeds from sale of securities	$ 879,900,000	1,200,000,000	$ 307,300,000
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
NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to Maturity	$ 1,261,800,000
Percentage of debt securities, held-to-maturity, portfolio	0.07
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	$ 26,900,000
Held to Maturity	3,000,000.0	4,100,000
Government Sponsored Agency | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to Maturity	0
Other Asset-Backed
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	2,800,000
Held to Maturity	677,000,000.0	804,300,000
Other Asset-Backed | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to Maturity	$ 0
Corporate Debt
Debt and Equity Securities, FV-NI [Line Items]
Percent of corporate debt portfolio	16.00%
Provision for credit losses	$ 0
Held to Maturity	588,000,000.0	$ 405,100,000
Corporate Debt | NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Held to Maturity	$ 0