Loans (Troubled Debt Restructurings) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	$ 17,524	$ 13,426	$ 15,911
Balance at Year-end	8,552	11,674	0
Commercial Portfolio Segment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	3,661	3,845	2,203
Balance at Year-end	192	2,161	0
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	2,432	0	13,708
Balance at Year-end	2,421	0	0
Commercial Real Estate, Buildings, Land and Other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	9,310	9,457	0
Balance at Year-end	4,922	9,393	0
Construction Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	1,017	0	0
Balance at Year-end	1,017	0	0
Total consumer real estate loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	0	124	0
Balance at Year-end	0	120	0
Consumer Loan [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	1,104	0	0
Balance at Year-end	$ 0	$ 0	$ 0