Noninterest Income - Client Investment Fees (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 221,371	$ 122,459	$ 438,628	$ 233,821
Sweep money market fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	26,952	17,178	53,496	29,500
Asset management fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6,956	5,730	13,628	11,088
Repurchase agreement fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	11,836	6,544	23,102	11,739
Client investment fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 45,744	$ 29,452	$ 90,226	$ 52,327