Other Borrowings - Additional Information (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Borrowings [Line Items]
Security repurchase agreement	$ 0	$ 0
FHLB collateral line of credit, maximum borrowings capacity	1,597,695,000
FHLB collateral line of credit, outstanding balance	48,000,000
FHLB collateral line of credit, remaining borrowing capacity	1,549,695,000
Loans designated as potential collateral under collateralized line of credit with FHLB	205,777,000
Investment securities designated as potential collateral under collateral line of credit	2,138,150,000
Repurchase investment securities sold and pledged as collateral under securities repurchase agreement	31,667,000
Collateralized line of credit with San Francisco Federal Reserve Bank, maximum borrowings capacity	140,921,000
Collateralized line of credit with San Francisco Federal Reserve Bank, outstanding balance	0
Collateralized line of credit with San Francisco Federal Reserve Bank, remaining borrowing capacity	140,921,000
Investment securities designated as potential collateral under collateral line of credit with San Francisco Federal Reserve Bank	15,677
Loans designated as potential collateral under collateral line of credit with San Francisco Federal Reserve Bank	262,663,000
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]
Collateralized borrowings	$ 17,041,000	$ 17,437,000