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, 2018	Sep. 30, 2017
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 1,386,853	$ 191,371
Additions	22,969	44,964
Goodwill, ending balance	1,409,822	236,335
Regional Banking
Goodwill [Roll Forward]
Goodwill, beginning balance	1,243,885	93,367
Additions	22,969	0
Goodwill, ending balance	1,266,854	93,367
Fixed Income
Goodwill [Roll Forward]
Goodwill, beginning balance	142,968	98,004
Additions	0	44,964
Goodwill, ending balance	$ 142,968	$ 142,968