Other Borrowings - Additional Information (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Borrowings [Line Items]
Security repurchase agreement	$ 0	$ 0
FHLB collateral line of credit, maximum borrowings capacity	2,063,815,000
FHLB collateral line of credit, outstanding balance	0
Loans designated as potential collateral under collateralized line of credit with FHLB	188,559,000
Investment securities designated as potential collateral under collateral line of credit	2,832,945,000
Repurchase investment securities sold and pledged as collateral under securities repurchase agreement	37,688,000
Collateralized line of credit with San Francisco Federal Reserve Bank, maximum borrowings capacity	142,272,000
Collateralized line of credit with San Francisco Federal Reserve Bank, outstanding balance	0
Investment securities designated as potential collateral under collateral line of credit with San Francisco Federal Reserve Bank	12,000
Loans designated as potential collateral under collateral line of credit with San Francisco Federal Reserve Bank	256,364,000
Unused correspondent banking lines of credit from commercial banks for federal funds	20,000,000
0.05% Fixed Rate Collateralized Borrowings [Member]
Schedule Of Borrowings [Line Items]
Other collateralized borrowings	$ 15,839,000	$ 17,437,000