Business Combinations - Additional Information (Detail)			6 Months Ended	9 Months Ended
	Oct. 28, 2015 USD ($) Branches	Oct. 03, 2014 USD ($) $ / shares shares	Jun. 30, 2014 USD ($) Bank	Sep. 30, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	Aug. 21, 2007 shares
Business Acquisition [Line Items]
Business acquisition, common stock issued to acquiree's shareholders | shares		6,575,550
Fixed share exchange ratio		0.9433
Aggregate ownership percentage		28.90%
Cash in-lieu of fractional shares		$ 6,823
Business combination share outstanding | shares				22,764,295	22,714,964	15,814,662
Assets				$ 4,021,628,000	$ 3,916,458,000
Bank Of America [Member] | Subsequent Event [Member]
Business Acquisition [Line Items]
Number of branches | Branches	3
Loans	$ 400,000
Deposits	$ 245,000,000
Percentage of premium on deposits	1.91%
North Valley Bancorp [Member]
Business Acquisition [Line Items]
Loans		499,327,000
Deposits		$ 801,956,000
Acquisition, completion date				Oct. 03, 2014
Closing stock price | $ / shares		$ 23.01
Consideration paid per share | $ / shares		$ 21.71
Cash in-lieu of fractional shares		$ 7,000
Business combination share outstanding | shares		22,714,964
Business combination consideration value		$ 151,310,000
Number of commercial banks | Bank			22
Assets			$ 935,000,000
Increase in net book value of land and buildings acquired		4,785,000
Recognized intangible related to acquisition		6,614,000
North Valley Bancorp [Member] | Junior subordinated debt [Member]
Business Acquisition [Line Items]
Discount on debt instrument		$ 6,664,000
North Valley Bancorp [Member] | Minimum [Member] | Junior subordinated debt [Member]
Business Acquisition [Line Items]
Amortization period of debt discount		18 years
North Valley Bancorp [Member] | Maximum [Member] | Junior subordinated debt [Member]
Business Acquisition [Line Items]
Amortization period of debt discount		21 years
North Valley Bancorp [Member] | Core deposit intangibles [Member]
Business Acquisition [Line Items]
Recognized intangible related to acquisition		$ 6,614,000
Percentage of acquired intangibles		0.97%
Finite lived intangible asset useful life		7 years