Loans (Troubled Debt Restructurings) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	$ 13,426	$ 15,911	$ 60,510
Balance at Year-end	11,674	0	58,484
Commercial Portfolio Segment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	3,845	2,203	4,026
Balance at Year-end	2,161	0	3,766
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	0	13,708	56,096
Balance at Year-end	0	0	54,330
Commercial Real Estate, Buildings, Land and Other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	9,457	0	0
Balance at Year-end	9,393	0	0
Construction Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	0	0	388
Balance at Year-end	0	0	388
Total consumer real estate loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at Restructure	124	0	0
Balance at Year-end	$ 120	$ 0	$ 0