Business Combinations - Summary of Estimated Fair Value Adjustments Resulting In Goodwill (Detail) - USD ($)	Oct. 03, 2014	Sep. 30, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Cash payments to North Valley Bancorp Shareholders	$ 6,823
Goodwill		$ 63,462,000	$ 63,462,000
North Valley Bancorp [Member]
Business Acquisition [Line Items]
Value of stock consideration paid to North Valley Bancorp Shareholders	151,303,000
Cash payments to North Valley Bancorp Shareholders	7,000
Cost basis net assets acquired	(98,040,000)
Other	280,000
Goodwill	47,943,000
North Valley Bancorp [Member] | Deferred income taxes [Member]
Business Acquisition [Line Items]
Fair Value Adjustments	6,293,000
North Valley Bancorp [Member] | Loans [Member]
Business Acquisition [Line Items]
Fair Value Adjustments	5,832,000
North Valley Bancorp [Member] | Premises and equipment [Member]
Business Acquisition [Line Items]
Fair Value Adjustments	(4,785,000)
North Valley Bancorp [Member] | Core deposit intangibles [Member]
Business Acquisition [Line Items]
Fair Value Adjustments	(6,283,000)
North Valley Bancorp [Member] | Junior subordinated debt [Member]
Business Acquisition [Line Items]
Fair Value Adjustments	$ (6,664,000)