Derivatives - Summary Information Regarding Derivatives (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2016
Customer interest rate swaps
Classified in Other Assets:
Interest rate	199.00%	193.00%
Classified in Other Liabilities:
Notional Amount		$ 85,796
Average Maturity (Years)		9 years
Weighted Average Fixed Rate		3.51%
Fair  Value		$ (947)
Interest rate		195.00%
Notional Amount	$ 919,460	$ 473,273
Average Maturity (Years)	9 years 2 months 12 days	9 years 10 months 24 days
Weighted Average Fixed Rate	3.78%	4.32%
Fair Value	$ 95,243	$ 25,905
3rd Party interest rate swaps
Classified in Other Assets:
Interest rate		195.00%
Classified in Other Liabilities:
Notional Amount	$ 919,460	$ 473,273
Average Maturity (Years)	9 years 2 months 12 days	9 years 10 months 24 days
Weighted Average Fixed Rate	3.78%	4.32%
Fair  Value	$ (95,243)	$ (25,905)
Interest rate	199.00%	193.00%
Notional Amount		$ 85,796
Average Maturity (Years)		9 years
Weighted Average Fixed Rate		3.51%
Fair Value		$ 947
Interest rate swap (cash flow hedge)
Classified in Other Liabilities:
Notional Amount	$ 30,000
Average Maturity (Years)	9 months 18 days
Weighted Average Fixed Rate	1.10%
Fair  Value	$ 206
Interest rate swap (cash flow hedge) | Interest rate swap (cash flow hedge)
Classified in Other Liabilities:
Notional Amount			$ 30,000
Notional Amount		$ 30,000
Average Maturity (Years)		1 year 6 months
Weighted Average Fixed Rate		1.10%
Fair Value		$ 271