Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) Grade	Dec. 31, 2017 USD ($) Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 5,043,272	$ 4,792,388
Loans	5,509,839	5,286,424
Loans	$ 1,106,999	$ 1,066,696
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.33	6.41
Loans	$ 5,043,272	$ 4,792,388
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.74	6.97
Loans	$ 1,528,028	$ 1,499,093
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.01	7.00
Loans	$ 4,402,840	$ 4,219,728
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.20	7.23
Loans	$ 1,106,999	$ 1,066,696
Risk grades 1-8 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.10	6.06
Loans	$ 4,737,862	$ 4,378,839
Risk grades 1-8 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	5.98	6.01
Loans	$ 1,288,490	$ 1,199,207
Risk grades 1-8 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.77	6.75
Loans	$ 4,071,883	$ 3,868,659
Risk grades 1-8 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.13	7.11
Loans	$ 1,077,422	$ 1,019,635
Risk grade 9 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans	$ 110,377	$ 170,285
Risk grade 9 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans	$ 44,181	$ 50,427
Risk grade 9 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans	$ 130,689	$ 151,487
Risk grade 9 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans	$ 10,873	$ 18,042
Risk grade 10 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans	$ 117,623	$ 99,260
Risk grade 10 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans	$ 48,115	$ 64,282
Risk grade 10 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans	$ 101,505	$ 129,391
Risk grade 10 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans	$ 17,237	$ 23,393
Risk grade 11 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans	$ 60,104	$ 97,818
Risk grade 11 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans	$ 67,279	$ 90,875
Risk grade 11 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans	$ 79,348	$ 62,602
Risk grade 11 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans	$ 1,467	$ 5,626
Risk grade 12 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans	$ 9,639	$ 38,633
Risk grade 12 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans	$ 65,591	$ 81,035
Risk grade 12 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans	$ 18,416	$ 7,589
Risk grade 12 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans	$ 0	$ 0
Risk grade 13 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans	$ 7,667	$ 7,553
Risk grade 13 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans	$ 14,372	$ 13,267
Risk grade 13 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans	$ 999	$ 0
Risk grade 13 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans	$ 0	$ 0