Junior Subordinated Debt - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Location Trust	Dec. 31, 2016 USD ($)	Oct. 03, 2014 USD ($)
Class of Stock [Line Items]
Number of subsidiary business trusts | Trust	5
Trust preferred securities amount	$ 62,900,000
Subordinated debenture, recorded fair value	$ 56,858,000	$ 56,667,000
Number of locations organized | Location	2
Maximum [Member]
Class of Stock [Line Items]
Consecutive payments of interest on debenture defer Period	5 years
North Valley Bancorp [Member]
Class of Stock [Line Items]
Number of locations acquired | Location	3
Acquisition, completion date	Oct. 03, 2014
North Valley Trust II [Member]
Class of Stock [Line Items]
Subordinated debenture, recorded fair value	$ 5,135,000		$ 5,006,000
North Valley Trust III [Member]
Class of Stock [Line Items]
Subordinated debenture, recorded fair value	4,041,000		3,918,000
North Valley Trust IV [Member]
Class of Stock [Line Items]
Subordinated debenture, recorded fair value	$ 6,444,000		$ 6,063,000