Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	$ 6,208	$ 15,911
Balance at period end	6,191	14,229
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	307	2,203
Balance at period end	293	2,171
Commercial and industrial, Energy [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	0	13,708
Balance at period end	0	12,058
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	5,901	0
Balance at period end	$ 5,898	$ 0