Securities (Narrative) (Details)	3 Months Ended
	Mar. 31, 2021 USD ($) investment	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss position | investment	991
Total fair value	$ 12,500,000,000
Total unrealized losses	257,700,000
Debt securities, unrealized losses	257,700,000		$ 31,300,000
Provision for credit losses	(30,000,000.0)	$ 60,800,000
Debt securities, held-to-maturity	17,136,400,000		$ 17,791,100,000
Debt securities, held-to-maturity, unrealized loss position, fair value	75,000,000
Held to maturity securities, transferred from available for sale	$ 0	0
Percentage of debt securities, held-to-maturity, portfolio	1		1
Gross proceeds from sale of securities	$ 0	280,000,000.0
Debt securities, available-for-sale, realized gain		700,000
Debt securities, available-for-sale, realized loss		700,000
Realized investment gains (losses)	$ 0
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		0.93
BBB
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	$ 272,800,000		$ 49,900,000
Percentage of debt securities, held-to-maturity, portfolio	0.02		0
NOT RATED
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	$ 1,100,200,000		$ 1,261,800,000
Percentage of debt securities, held-to-maturity, portfolio	0.07		0.07
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	$ 160,200,000
Debt securities, unrealized losses	160,200,000		$ 26,900,000
Debt securities, held-to-maturity	53,600,000		3,000,000.0
Government Sponsored Agency | BBB
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]
Debt securities, unrealized losses	4,500,000		2,800,000
Debt securities, held-to-maturity	706,200,000		677,000,000.0
Other Asset-Backed | BBB
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]
Debt securities, unrealized losses	$ 2,800,000		100,000
Percent of corporate debt portfolio	17.00%
Provision for credit losses	$ 0	$ 200,000
Debt securities, available-for-sale, allowance for credit loss	0		0
Debt securities, held-to-maturity	916,700,000		588,000,000.0
Corporate Debt | BBB
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, held-to-maturity	0		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	456,100,000		454,600,000
Other | BBB
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	456,100,000		454,600,000
U.S. Government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, unrealized losses	10,700,000
Debt securities, held-to-maturity	100,000,000.0		90,000,000.0
U.S. Government | BBB
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