Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($)	Dec. 31, 2016 USD ($) Branches Segment Trust Location Business	Dec. 31, 2015 USD ($)
Significant Of Accounting Policies [Line Items]
Number of subsidiary business trusts | Trust		5
Company's investments in the trusts		$ 1,702,000
Number of business segment | Segment		1
Trading securities		$ 0	$ 0
Recognized OTTI losses		$ 0	$ 0
Loans contractual past due		90 days
TDR loans payment period in nonaccrual status		6 months
Tri Counties Bank [Member]
Significant Of Accounting Policies [Line Items]
Number of locations acquired | Location		3
Number of traditional operating branches | Branches		58
Number of in-store operating branches | Branches		10
California [Member]
Significant Of Accounting Policies [Line Items]
Number of counties | Business		26
Home Equity Lines of Credit [Member]
Significant Of Accounting Policies [Line Items]
Increase in allowance for loan losses due to change in methodology	$ 859,000
Home Equity Loans [Member]
Significant Of Accounting Policies [Line Items]
Increase in allowance for loan losses due to change in methodology	$ 459,000
Furniture and Equipment [Member] | Minimum [Member]
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms		3 years
Furniture and Equipment [Member] | Maximum [Member]
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms		10 years
Land Improvements and Buildings [Member] | Minimum [Member]
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms		15 years
Land Improvements and Buildings [Member] | Maximum [Member]
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms		40 years