Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	$ 15,911	$ 13,743
Balance at period end	843	13,101
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	2,203	784
Balance at period end	843	643
Commercial and industrial, Energy [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	13,708	12,959
Balance at period end	$ 0	$ 12,458