Deposit Accounts - Deposit accounts and weighted average interest rates (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Transaction accounts
Noninterest-bearing checking	$ 2,226,848	$ 1,185,768
Interest-bearing checking	365,193	182,893
Money market	2,181,571	1,100,787
Savings	227,436	101,574
Total transaction accounts	5,001,048	2,571,022
Certificates of deposit accounts
Less than 100,000	192,409	121,148
$100,000 through $250,000	369,748	153,103
Greater than $250,000	522,663	300,308
Total certificates of deposit accounts	1,084,820	574,559
Total deposits	$ 6,085,868	$ 3,145,581
Transaction accounts
Noninterest-bearing checking (as a percent)	0.00%	0.00%
Interest-bearing checking (as a percent)	0.13%	0.11%
Money market (as a percent)	0.48%	0.34%
Savings (as a percent)	0.13%	0.14%
Total transaction accounts (as a percent)	0.21%	0.16%
Certificates of deposit accounts
Less than $100,000 (as a percent)	0.85%	0.74%
$100,000 through $250,000 (as a percent)	1.01%	0.82%
Greater than $250,000 (as a percent)	1.26%	0.74%
Total certificates of deposit accounts (as a percent)	1.10%	0.76%
Total deposits (as a percent)	0.33%	0.27%