Deposit Accounts - Deposit accounts and weighted average interest rates (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Transaction accounts
Noninterest-bearing checking	$ 3,495,737	$ 2,226,876
Interest-bearing checking	526,088	365,193
Money market	2,975,578	2,181,571
Savings	250,271	227,436
Total transaction accounts	7,247,674	5,001,076
Certificates of deposit accounts
250,000 or less	569,877	562,147
Greater than $250,000	840,800	522,663
Total certificates of deposit accounts	1,410,677	1,084,810
Total deposits	$ 8,658,351	$ 6,085,886
Transaction accounts
Noninterest-bearing checking (as a percent)	0.00%	0.00%
Interest-bearing checking (as a percent)	0.38%	0.13%
Money market (as a percent)	0.89%	0.48%
Savings (as a percent)	0.14%	0.13%
Total transaction accounts (as a percent)	0.37%	0.21%
Certificates of deposit accounts
Less than $100,000 (as a percent)	1.44%	0.96%
Greater than $250,000 (as a percent)	2.12%	1.26%
Total certificates of deposit accounts (as a percent)	1.84%	1.10%
Total deposits (as a percent)	0.63%	0.33%