Deposit Accounts - The aggregate annual maturities of certificates of deposit accounts (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Balance
Within 3 months	$ 318,794
4 to 6 months	250,026
7 to 12 months	279,192
13 to 24 months	175,005
25 to 36 months	29,270
37 to 60 months	22,936
Over 60 months	9,597
Total certificates of deposit accounts	$ 1,084,820	$ 574,559
Weighted Average Interest Rate
Within 3 months (as a percent)	0.93%
4 to 6 months (as a percent)	1.07%
7 to 12 months (as a percent)	1.18%
13 to 24 months (as a percent)	1.24%
25 to 36 months (as a percent)	1.33%
37 to 60 months (as a percent)	1.44%
Over 60 months (as a percent)	1.13%
Total certificates of deposit accounts (as a percent)	1.10%	0.76%