Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) Branches Segment Location Trust Business	Dec. 31, 2016 USD ($)	Jun. 30, 2017 USD ($)
Significant Of Accounting Policies [Line Items]
Number of subsidiary business trusts | Trust		5
Company's investments in the trusts	$ 1,706,000	$ 1,706,000
Number of business segment | Segment		1
Trading securities			$ 0	$ 0
Sale of classified investment securities	24,796,000
Proceeds from sale of investment securities	25,757,000
Gain from sale of classified investment securities	961,000	$ 961,000
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		57
Number of in-store operating branches | Branches		9
California [Member]
Significant Of Accounting Policies [Line Items]
Number of counties | Business		26
ASU 2016-09 [Member]
Significant Of Accounting Policies [Line Items]
Recognition of excess tax benefits	150,000	$ 847,000
Reclassifications [Member] | Mortgage Loans on Real Estate [Member] | Residential 1-4 Family [Member]
Significant Of Accounting Policies [Line Items]
Loans	60,957,000	60,957,000
Reclassifications [Member] | Consumer [Member] | Home Equity Loans [Member]
Significant Of Accounting Policies [Line Items]
Loans	$ 5,620,000	$ 5,620,000
Minimum [Member] | Parent [Member] | Reclassifications [Member]
Significant Of Accounting Policies [Line Items]
Total risk based capital ratios, increase percentage	0.10%
Tier 1 capital ratios, increase percentage	0.10%
Tier 1 common equity ratios, increase percentage	0.10%
Minimum [Member] | Tri Countries Bank [Member] | Reclassifications [Member]
Significant Of Accounting Policies [Line Items]
Total risk based capital ratios, increase percentage	0.10%
Tier 1 capital ratios, increase percentage	0.10%
Tier 1 common equity ratios, increase percentage	0.10%
Maximum [Member] | Parent [Member] | Reclassifications [Member]
Significant Of Accounting Policies [Line Items]
Total risk based capital ratios, increase percentage	0.20%
Tier 1 capital ratios, increase percentage	0.20%
Tier 1 common equity ratios, increase percentage	0.20%
Maximum [Member] | Tri Countries Bank [Member] | Reclassifications [Member]
Significant Of Accounting Policies [Line Items]
Total risk based capital ratios, increase percentage	0.20%
Tier 1 capital ratios, increase percentage	0.20%
Tier 1 common equity ratios, increase percentage	0.20%
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