Loans (Troubled Debt Restructurings) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Modifications [Line Items]
Balance at Restructure	$ 60,510	$ 91,418	$ 709
Balance at Year-end	58,484	44,863	536
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Balance at Restructure	4,026	2,148	709
Balance at Year-end	3,766	1,022	536
Commercial and Industrial, Energy [Member]
Financing Receivable, Modifications [Line Items]
Balance at Restructure	56,096	87,572	0
Balance at Year-end	54,330	43,841	0
Commercial Real Estate, Buildings, Land and Other [Member]
Financing Receivable, Modifications [Line Items]
Balance at Restructure	0	1,455	0
Balance at Year-end	0	0	0
Construction Loans [Member]
Financing Receivable, Modifications [Line Items]
Balance at Restructure	388	243	0
Balance at Year-end	$ 388	$ 0	$ 0