Deposits - Summary of Company's Deposits by Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Banking and Thrift [Abstract]
Noninterest-bearing demand deposits	$ 1,478,837	$ 1,236,685
Interest-bearing demand deposits	1,375,820	1,454,097
Interest-bearing NOW accounts	474,737	363,191
Savings and money market accounts	2,919,708	3,013,237
Time deposits	3,237,174	2,606,717
Total deposits	9,486,276	8,673,927
Time deposits $100,000 and greater	2,410,046	1,983,445
Time deposits greater than $250,000	$ 1,333,848	$ 1,078,702