Loans - Average Recorded Investment In Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 123,227	$ 95,709	$ 130,688	$ 95,146
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	15,307	21,347	24,791	23,867
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	92,380	67,008	93,001	63,860
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	13,867	5,966	11,376	6,266
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	0	0	0	0
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	860	1,376	978	1,135
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 813	$ 12	$ 542	$ 18