Other Borrowings - Additional Information (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Debt Disclosure [Abstract]
Security repurchase agreement	$ 0	$ 0
Other collateralized borrowings	9,096,000	9,276,000
Repurchase investment securities sold and pledged as collateral under securities repurchase agreement	9,096,000
FHLB collateral line of credit, maximum borrowings capacity	978,743,000
FHLB collateral line of credit, outstanding balance	0
FHLB collateral line of credit, remaining borrowing capacity	978,743,000
Loans designated as potential collateral under collateralized line of credit with FHLB	1,567,140,000
Collateralized line of credit with San Francisco Federal Reserve Bank, maximum borrowings capacity	138,447,000
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	138,447,000
Investment securities designated as potential collateral under collateral line of credit with San Francisco Federal Reserve Bank	427,000
Loans designated as potential collateral under collateral line of credit with San Francisco Federal Reserve Bank	205,883,000
Unused correspondent banking lines of credit from commercial banks for federal funds	$ 15,000,000