Investment Securities (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Realized gains on available-for-sale securities excluding losses from other than temporary impairment ("OTTI")	$ 12,500,000	$ 1,100,000
Cash and interest bearing deposits	3,477,800,000	1,190,100,000	$ 1,695,500,000
Equity securities carried at fair value with changes recorded in net income, amortized cost	400,000		48,200,000
Equity securities	0.0		47,200,000
Equity securities carried at fair value with changes recorded in net income, unrealized losses	400,000		1,000,000.0
Other than temporary impairment losses	0	$ 0
Pledged securities carrying value	$ 3,167,000,000.0		$ 50,400,000