Deposit Accounts - Deposit accounts and weighted average interest rates (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Transaction accounts
Noninterest-bearing checking	$ 1,185,672	$ 711,771
Interest-bearing checking	182,893	134,999
Money market	1,100,787	743,871
Savings	101,574	83,507
Total transaction accounts	2,570,926	1,674,148
Certificates of deposit accounts
Less than 100,000	121,148	126,704
$100,000 through $250,000	153,103	166,397
Greater than $250,000	300,308	227,874
Total certificates of deposit accounts	574,559	520,975
Total deposits	$ 3,145,485	$ 2,195,123
Transaction accounts
Noninterest-bearing checking (as a percent)	0.00%	0.00%
Interest-bearing checking (as a percent)	0.11%	0.11%
Money market (as a percent)	0.34%	0.35%
Savings (as a percent)	0.14%	0.15%
Total transaction accounts (as a percent)	0.16%	0.17%
Certificates of deposit accounts
Less than $100,000 (as a percent)	0.74%	0.79%
$100,000 through $250,000 (as a percent)	0.82%	0.91%
Greater than $250,000 (as a percent)	0.74%	0.72%
Total certificates of deposit accounts (as a percent)	0.76%	0.80%
Total deposits (as a percent)	0.27%	0.32%