Allowance for Credit Losses (Narrative) (Details) - USD ($)		3 Months Ended			9 Months Ended
	Jan. 01, 2020	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Allowance for credit loss		$ 215,400,000		$ 105,700,000	$ 215,400,000	$ 105,700,000	$ 210,200,000	$ 104,500,000	$ 110,800,000		$ 112,600,000
Stockholders equity		(11,579,100,000)	$ (10,879,700,000)	(10,811,300,000)	(11,579,100,000)	(10,811,300,000)	(11,382,200,000)	(11,091,000,000.0)	(10,805,500,000)	$ (10,616,200,000)	(10,508,300,000)
Credit loss expense (reversal) and available-for-sale, allowance for credit loss		500,000		(7,000,000.0)	127,500,000	(13,500,000)
Net (charge-offs) recoveries		400,000		600,000	$ 2,300,000	3,000,000.0
Time period used for projecting future conditions to estimate credit losses					2 years
Interest income that would have been recorded for nonperforming loans				1,800,000		5,700,000
Financing receivable accrued interest income		1,100,000			$ 3,500,000
Other-than-temporary impairment loss, debt securities, portion recognized in earnings		0	4,400,000	100,000	0	300,000
Other financial assets, allowance for credit loss		700,000			700,000		1,300,000	0
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	$ 13,700,000							(2,200,000)
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		$ (12,118,300,000)	$ (11,828,100,000)	$ (11,441,200,000)	$ (12,118,300,000)	$ (11,441,200,000)	$ (11,988,400,000)	(11,656,700,000)	$ (11,225,500,000)	$ (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