Noninterest Income and Expense - Components of Noninterest Expense (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Other Noninterest Income And Expense [Line Items]
Base salaries, net of deferred loan origination costs									$ 54,589	$ 53,169	$ 46,822
Incentive compensation									9,227	8,872	6,964
Benefits and other compensation costs									19,114	18,683	17,619
Total salaries and benefits expense									82,930	80,724	71,405
Occupancy									10,894	10,139	10,126
Data processing and software									10,448	8,846	7,670
Equipment									7,141	6,597	5,997
ATM & POS network charges									4,752	4,999	4,190
Advertising									4,101	3,829	3,992
Professional fees									3,745	5,409	4,545
Telecommunications									2,713	2,749	3,007
Assessments									1,676	2,105	2,572
Operational losses									1,394	1,564	737
Intangible amortization									1,389	1,377	1,157
Postage									1,296	1,603	1,296
Courier service									1,035	998	1,154
Change in reserve for unfunded commitments									445	244	330
Foreclosed assets expense									231	266	490
Provision for foreclosed asset losses									162	140	502
Legal settlement									1,469	1,450
Merger & acquisition expense									530	784	586
Miscellaneous other									12,142	12,174	11,085
Total other noninterest expense									64,094	65,273	59,436
Total noninterest expense	$ 38,076	$ 37,222	$ 35,904	$ 35,822	$ 36,563	$ 37,416	$ 38,267	$ 33,751	147,024	145,997	130,841
Total merger expense									530	784	586
North Valley Bancorp [Member]
Schedule Of Other Noninterest Income And Expense [Line Items]
Base salaries, net of loan origination costs										187
Data processing and software											108
Professional fees									513	342	120
Miscellaneous other									$ 17	$ 255	$ 358