Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	$ 15,911	$ 11,262
Balance at period end	14,229	11,212
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	2,203	0
Balance at period end	2,171	0
Commercial and industrial, Energy [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	13,708	11,262
Balance at period end	$ 12,058	$ 11,212