Allowance for Credit Losses (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Mar. 31, 2020	Jan. 01, 2020	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss	$ 210,200,000	$ 110,800,000	$ 210,200,000	$ 110,800,000	$ 104,500,000	$ 147,200,000		$ 114,500,000	$ 112,600,000
Stockholders' equity	11,382,200,000	10,805,500,000	11,382,200,000	10,805,500,000	11,091,000,000.0	10,879,700,000		10,616,200,000	10,508,300,000
Provision for credit losses	66,200,000	(6,500,000)	127,000,000.0	(6,500,000)
Credit loss expense (reversal) and available-for-sale, allowance for credit loss	66,000,000.0	(6,500,000)	127,000,000.0	(6,500,000)
Net (charge-offs) recoveries	2,600,000	1,200,000	$ 1,900,000	2,400,000
Time period used for projecting future conditions to estimate credit losses			2 years
Interest income that would have been recorded for nonperforming loans		2,000,000.0		3,900,000
Financing receivable accrued interest income	1,100,000		$ 2,400,000
Other-than-temporary impairment loss, debt securities, portion recognized in earnings	0	200,000	0	200,000	4,400,000
Other financial assets, allowance for credit loss	1,300,000		1,300,000		0	1,100,000
Other Financial Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable accrued interest income	0	0	0	0
Cumulative Effect Adjustment
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss					(2,200,000)		$ 13,700,000
Stockholders' equity					(10,100,000)
Other financial assets, allowance for credit loss					400,000
Retained Earnings
Financing Receivable, Allowance for Credit Loss [Line Items]
Stockholders' equity	$ 11,988,400,000	$ 11,225,500,000	$ 11,988,400,000	$ 11,225,500,000	11,656,700,000	$ 11,828,100,000		$ 10,972,900,000	$ 10,776,800,000
Retained Earnings | Cumulative Effect Adjustment
Financing Receivable, Allowance for Credit Loss [Line Items]
Stockholders' equity					$ (10,100,000)		$ (10,100,000)