Noninterest Income and Expense - Components of Noninterest Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Schedule Of Other Noninterest Income And Expense [Line Items]
Base salaries, net of deferred loan origination costs	$ 13,962				$ 13,390
Incentive compensation	2,452				2,198
Benefits and other compensation costs	5,238				5,305
Total salaries and benefits expense	21,652				20,893
Occupancy	2,681				2,692
Data processing and software	2,514				2,396
Equipment	1,551				1,723
ATM and POS network charges	1,226				853
Advertising	838				967
Professional fees	773				766
Telecommunications	701				643
Change in reserve for unfunded commitments	700				15
Merger and acquisition expense	476
Assessments	430				405
Postage	358				404
Intangible amortization	339				359
Operational losses	294				435
Courier service	267				254
Provision for (reversal of) foreclosed asset losses	90				(66)
Foreclosed assets expense	24				38
Other miscellaneous expense	3,248				3,045
Total other noninterest expense	16,510				14,929
Total noninterest expense	38,162	$ 38,076	$ 37,222	$ 35,904	$ 35,822
Total merger and acquisition expense	476
North Valley Bancorp [Member]
Schedule Of Other Noninterest Income And Expense [Line Items]
Professional fees	355
Advertising and marketing	8
Other miscellaneous expense	$ 112