Deposits - Summary of Deposits Including Accrued Interest Payable (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Banking and Thrift [Abstract]
Non-interest-bearing accounts, Amount	$ 1,151,362	$ 756,513
Interest-bearing checking accounts, Amount	2,350,106	1,954,358
Money market deposit accounts, Amount	569,680	363,656
Savings accounts, Amount	877,177	661,167
Time deposits, Amount	866,244	607,104
Total deposits	$ 5,814,569	$ 4,342,798
Non-interest-bearing accounts, Weighted Average Cost	0.00%	0.00%
Interest-bearing checking accounts, Weighted Average Cost	0.51%	0.32%
Money market deposit accounts, Weighted Average Cost	0.66%	0.33%
Savings accounts, Weighted Average Cost	0.12%	0.04%
Time deposits, Weighted Average Cost	1.50%	1.18%
Weighted Average Cost	0.51%	0.34%