Allowance for Credit Losses (Narrative) (Details) - USD ($)		12 Months Ended
	Jan. 01, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss		$ 190,700,000	$ 104,500,000	$ 112,600,000	$ 131,200,000
Decrease to stockholders equity		$ (11,688,300,000)	(11,091,000,000.0)	(10,508,300,000)	(10,216,200,000)
Time period used for projecting future conditions to estimate credit losses		2 years
Other-than-temporary impairment loss, debt securities, portion recognized in earnings	$ 4,400,000	$ 0	300,000	500,000
Other financial assets, allowance for credit loss		800,000	0
Other Financial Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, accrued interest, writeoff		0	0	0
Cumulative Effect Adjustment
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss			(2,200,000)
Decrease to stockholders equity			10,100,000		4,500,000
Cumulative Effect Adjustment | Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss			13,700,000
Retained Earnings
Financing Receivable, Allowance for Credit Loss [Line Items]
Decrease to stockholders equity		$ (12,207,700,000)	(11,656,700,000)	$ (10,776,800,000)	(9,685,100,000)
Retained Earnings | Cumulative Effect Adjustment
Financing Receivable, Allowance for Credit Loss [Line Items]
Decrease to stockholders equity			10,100,000		$ 4,500,000
Retained Earnings | Cumulative Effect Adjustment | Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Line Items]
Decrease to stockholders equity			$ 10,100,000