Other Borrowings - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Borrowings [Line Items]
Security repurchase agreement	$ 0	$ 0
FHLB collateral line of credit, maximum borrowings capacity	1,365,325,000
FHLB collateral line of credit, outstanding balance	104,729,000
FHLB collateral line of credit, remaining borrowing capacity	1,260,596,000
Loans designated as potential collateral under collateralized line of credit with FHLB	67,325,000
Investment securities designated as potential collateral under collateral line of credit	1,992,980,000
Repurchase investment securities sold and pledged as collateral under securities repurchase agreement	33,531,000
Collateralized line of credit with San Francisco Federal Reserve Bank, maximum borrowings capacity	134,660,000
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	134,660,000
Investment securities designated as potential collateral under collateral line of credit with San Francisco Federal Reserve Bank	17,000
Loans designated as potential collateral under collateral line of credit with San Francisco Federal Reserve Bank	245,532,000
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	$ 17,437,000	$ 17,493,000