Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	$ 13,743	$ 64,755
Balance at period end	13,101	22,980
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	784	510
Balance at period end	643	505
Commercial and industrial, Energy [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	12,959	62,546
Balance at period end	12,458	20,795
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	0	1,456
Balance at period end	0	1,456
Construction Loans [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	0	243
Balance at period end	$ 0	$ 224