Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	$ 13,425	$ 15,911
Balance at period end	11,805	0
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	3,845	2,203
Balance at period end	2,188	0
Commercial and industrial, Energy [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	0	13,708
Balance at period end	0	0
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	9,456	0
Balance at period end	9,494	0
Consumer Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	124	0
Balance at period end	$ 123	$ 0