Allowance for Credit Losses (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jan. 01, 2020	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss	$ 147,200,000	$ 114,500,000	$ 104,500,000
Stockholders' equity	10,879,700,000	10,616,200,000	11,091,000,000.0		$ 10,508,300,000
Provision for credit losses	60,800,000	0
Credit loss expense (reversal) and available-for-sale, allowance for credit loss	61,000,000.0	0
Change In Present Value, Expense (Reversal)	700,000	(1,200,000)
Net (Charge-Offs) Recoveries	$ (700,000)	1,200,000
Time period used for projecting future conditions to estimate credit losses	2 years
Interest income that would have been recorded for nonperforming loans		1,900,000
Financing receivable accrued interest income	$ 1,300,000
Other-than-temporary impairment loss, debt securities, portion recognized in earnings			4,400,000
Debt securities, held-to-maturity, allowance for credit loss	9,500,000		0
Other financial assets, allowance for credit loss	1,100,000		0
Other Financial Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable accrued interest income	0	0
Cumulative Effect Adjustment
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss			(2,200,000)	$ 13,700,000
Debt securities, held-to-maturity, allowance for credit loss			6,600,000
Other financial assets, allowance for credit loss			400,000
Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss		114,500,000	102,300,000		112,600,000
Debt securities, held-to-maturity, allowance for credit loss		0	6,600,000		0
Other financial assets, allowance for credit loss		0	400,000		0
Retained Earnings
Financing Receivable, Allowance for Credit Loss [Line Items]
Stockholders' equity	$ 11,828,100,000	$ 10,972,900,000	$ 11,656,700,000		$ 10,776,800,000
Retained Earnings | Cumulative Effect Adjustment
Financing Receivable, Allowance for Credit Loss [Line Items]
Stockholders' equity				$ (10,100,000)