Investment and Equity Securities - Amortized Cost and Fair Value of Held to Maturity Securities Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]
Due within twelve months, Estimated Fair Value	$ 1,675
Due after one but within five years, Estimated Fair Value	2,282
Due after five but within ten years, Estimated Fair Value	10,726
Due after ten years, Estimated Fair Value	14,599
Total Securities held for maturity, Estimated Fair Value	$ 29,282	$ 29,600
Due within twelve months, Book Yield	1.91%
Due after one but within five years, Book Yield	2.28%
Due after five but within ten years, Book Yield	4.93%
Due after ten years, Book Yield	2.64%
Total Securities held for maturity, Book Yield	3.43%
U.S. Government Agencies [Member]
Schedule Of Held To Maturity Securities [Line Items]
Due after one but within five years, Estimated Fair Value	$ 253
Total Securities held for maturity, Estimated Fair Value	$ 253	470
Due after one but within five years, Book Yield	2.67%
Total Securities held for maturity, Book Yield	2.67%
State and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Due within twelve months, Estimated Fair Value	$ 1,675
Due after one but within five years, Estimated Fair Value	2,029
Due after ten years, Estimated Fair Value	14,599
Total Securities held for maturity, Estimated Fair Value	$ 18,303	19,130
Due within twelve months, Book Yield	1.91%
Due after one but within five years, Book Yield	2.23%
Due after ten years, Book Yield	2.64%
Total Securities held for maturity, Book Yield	2.52%
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Due after five but within ten years, Estimated Fair Value	$ 10,726
Total Securities held for maturity, Estimated Fair Value	$ 10,726	$ 10,000
Due after five but within ten years, Book Yield	4.93%
Total Securities held for maturity, Book Yield	4.93%