Goodwill and Other Intangible Assets - Summary Of Gross Goodwill And Accumulated Impairment Losses And Write-Offs Detailed By Reportable Segments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Goodwill [Roll Forward]
Goodwill, Beginning balance	$ 1,432,787	$ 1,432,787
Additions	77,644	0
Goodwill, Ending balance	1,510,431	1,432,787
Regional Banking
Goodwill [Roll Forward]
Goodwill, Beginning balance	1,289,819	1,289,819
Additions	77,644	0
Goodwill, Ending balance	1,367,463	1,289,819
Fixed Income
Goodwill [Roll Forward]
Goodwill, Beginning balance	142,968	142,968
Additions	0	0
Goodwill, Ending balance	$ 142,968	$ 142,968