Loans - Average Recorded Investment In Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 102,283	$ 124,650
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	26,393	23,809
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	68,101	67,615
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	6,660	31,985
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	0	770
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	1,102	471
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 27	$ 0