Noninterest Income - Client Investment Fees (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 205,905	$ 130,437	$ 644,533	$ 364,258
Sweep money market fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	26,202	21,105	79,698	50,605
Asset management fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	7,256	6,358	20,883	17,447
Repurchase agreement fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	13,221	8,802	36,324	20,540
Client investment fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 46,679	$ 36,265	$ 136,905	$ 88,592