Noninterest Income and Expense - Components of Noninterest Expense (Detail) $ in Thousands	3 Months Ended							9 Months Ended
	Sep. 30, 2017 USD ($) Staff	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($) Staff	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2017 USD ($) Staff	Sep. 30, 2016 USD ($) Staff
Schedule Of Other Noninterest Income And Expense [Line Items]
Base salaries, net of deferred loan origination costs	$ 13,600				$ 13,419			$ 40,647	$ 39,095
Incentive compensation	2,609				2,798			6,980	7,008
Benefits and other compensation costs	4,724				4,643			14,693	14,067
Total salaries and benefits expense	20,933				20,860			62,320	60,170
Occupancy	2,799				2,667			8,196	7,504
Equipment	1,816				1,607			5,344	4,837
Data processing and software	2,495				2,068			7,332	6,266
ATM & POS network charges	1,425				1,915			3,353	3,923
Telecommunications	716				702			2,027	2,085
Postage	325				381			1,058	1,186
Courier service	235				280			752	816
Advertising	1,039				1,049			3,173	3,021
Assessments	427				654			1,252	1,864
Operational losses	301				497			1,166	1,006
Professional fees	901				1,018			2,357	3,183
Foreclosed assets expense	41				37			117	197
Provision for foreclosed asset losses	134				8			162	40
Change in reserve for unfunded commitments	390				25			270	433
Intangible amortization	339				359			1,050	1,017
Merger expense									784
Litigation contingent liability									1,450
Other	2,906				3,289			9,019	9,652
Total other noninterest expense	16,289				16,556			46,628	49,264
Total noninterest expense	$ 37,222	$ 35,904	$ 35,822	$ 36,563	$ 37,416	$ 38,267	$ 33,751	$ 108,948	109,434
Total merger expense									$ 784
Average full time equivalent staff | Staff	993				1,022			1,005	996
Noninterest expense to revenue (FTE)	64.60%				69.40%			64.50%	69.00%
North Valley Bancorp [Member]
Schedule Of Other Noninterest Income And Expense [Line Items]
Base salaries (outside temporary help)									$ 187
Equipment									35
Professional fees									342
Advertising and marketing									114
Other									$ 106