Business Combinations - Additional Information (Detail) (USD $)	0 Months Ended					6 Months Ended	9 Months Ended
	Oct. 03, 2014	Oct. 03, 2014	Sep. 30, 2014	Dec. 31, 2013	Aug. 21, 2007	Jun. 30, 2014 North Valley Bancorp [Member] Bank	Sep. 30, 2014 North Valley Bancorp [Member]	Oct. 03, 2014 North Valley Bancorp [Member]
Business Acquisition [Line Items]
Acquisition, completion date							Oct. 03, 2014
Business acquisition, common stock issued to acquiree's shareholders	6,575,550
Fixed share exchange ratio	0.9433
Aggregate ownership percentage		28.90%
Closing stock price								$ 23.01
Consideration paid per share								$ 21.71
Cash in-lieu of fractional shares	$ 6,823,000
Business combination share outstanding			16,139,414	16,076,662	15,814,662			22,714,964
Business combination consideration value								151,310,000
Number of commercial banks						22
Assets			$ 2,794,943,000	$ 2,744,066,000		$ 935,000,000