Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	$ 11,262	$ 62,808
Balance at period end	11,212	61,347
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	0	19
Balance at period end	0	17
Commercial and industrial, Energy [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	11,262	62,546
Balance at period end	11,212	61,095
Construction Loans [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	0	243
Balance at period end	$ 0	$ 235