Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) Office	Jun. 30, 2017	Jun. 30, 2018 USD ($) Office Segment Location Trust Branch Business	Jun. 30, 2017	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)
Significant Of Accounting Policies [Line Items]
Number of subsidiary business trusts | Trust			5
Number of loan production offices | Office	2		2
Company's investments in the trusts	$ 1,710,000		$ 1,710,000
Number of business segment | Segment			1
Marketable securities recorded	$ 2,868,000		$ 2,868,000		$ 2,938,000
Unrealized losses recognized in condensed consolidated statements of income			70,000
OTTI losses due to debt securities			$ 0		$ 0
Loans contractual past due			90 days
TDR loans payment period in nonaccrual status			6 months
Federal statutory income tax rate	21.00%	35.00%	21.00%	35.00%	35.00%
Tri Counties Bank [Member]
Significant Of Accounting Policies [Line Items]
Number of locations acquired | Location			3
Number of traditional operating branches | Branch			57
Number of in-store operating branches | Branch			9
California [Member]
Significant Of Accounting Policies [Line Items]
Number of counties | Business			26
ASU 2016-02 [Member] | Assets, Total [Member]
Significant Of Accounting Policies [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			$ 24,000,000
ASU 2016-02 [Member] | Liabilities, Total [Member]
Significant Of Accounting Policies [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			24,000,000
ASU 2016-01 [Member] | Deferred Tax Assets [Member]
Significant Of Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle cumulative effect of change on equity or net assets						$ (18,000)
ASU 2016-01 [Member] | Retained Earnings [Member]
Significant Of Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle cumulative effect of change on equity or net assets						(62,000)
ASU 2018-02 [Member] | Retained Earnings [Member]
Significant Of Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle cumulative effect of change on equity or net assets						1,093,000
ASU 2018-02 [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Significant Of Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle cumulative effect of change on equity or net assets						$ (1,093,000)
Trading Debt Securities [Member]
Significant Of Accounting Policies [Line Items]
Trading debt securities	$ 0		$ 0		$ 0
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