Other Borrowings - Additional Information (Detail) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Security repurchase agreement	$ 0	$ 0
FHLB collateral line of credit, maximum borrowings capacity	1,303,462,000
FHLB collateral line of credit, outstanding balance	85,737,000
FHLB collateral line of credit, remaining borrowing capacity	1,217,725,000
Loans designated as potential collateral under collateralized line of credit with FHLB	70,534,000
Investment securities designated as potential collateral under collateral line of credit	1,919,515,000
Other collateralized borrowings	12,993,000	$ 17,493,000
Repurchase investment securities sold and pledged as collateral under securities repurchase agreement	27,150,000
Collateralized line of credit with San Francisco Federal Reserve Bank, maximum borrowings capacity	130,985,000
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	130,985,000
Investment securities designated as potential collateral under collateral line of credit with San Francisco Federal Reserve Bank	18,000
Loans designated as potential collateral under collateral line of credit with San Francisco Federal Reserve Bank	232,290,000
Unused correspondent banking lines of credit from commercial banks for federal funds	$ 20,000,000