Loans - Schedule of Modified TDRs (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) SecurityLoan	Jun. 30, 2016 USD ($) SecurityLoan	Jun. 30, 2017 USD ($) SecurityLoan	Jun. 30, 2016 USD ($) SecurityLoan
Financing Receivable, Modifications
Post-modification outstanding recorded investment	$ 17,892	$ 135,524	$ 28,398	$ 165,283
Troubled Debt Restructurings
Financing Receivable, Modifications
Total TDRs	28,400	165,300
TDRs in accrual status	22,800	126,600
TDRs in non-accrual status	$ 5,600	$ 38,700
TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	124	151	206	236
Pre-modification outstanding recorded investment	$ 18,453	$ 119,083	$ 29,206	$ 157,121
Post-modification outstanding recorded investment	$ 17,892	$ 135,524	$ 28,398	$ 165,283
Commercial Loans | Commercial real estate- construction | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	1	0	1	1
Pre-modification outstanding recorded investment	$ 275	$ 0	$ 275	$ 28
Post-modification outstanding recorded investment	$ 275	$ 0	$ 275	$ 26
Commercial Loans | Commercial real estate- owner-occupied | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	1	3	2	5
Pre-modification outstanding recorded investment	$ 32	$ 794	$ 1,730	$ 1,069
Post-modification outstanding recorded investment	$ 31	$ 785	$ 1,698	$ 1,050
Commercial Loans | Commercial real estate- non-owner-occupied | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	6	6	11	10
Pre-modification outstanding recorded investment	$ 2,721	$ 10,777	$ 4,409	$ 11,516
Post-modification outstanding recorded investment	$ 2,795	$ 10,047	$ 4,465	$ 10,749
Commercial Loans | Commercial and industrial | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	24	16	33	28
Pre-modification outstanding recorded investment	$ 9,028	$ 19,496	$ 9,254	$ 24,018
Post-modification outstanding recorded investment	$ 8,551	$ 19,353	$ 8,747	$ 23,632
Commercial Loans | Energy-related | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	0	19	0	25
Pre-modification outstanding recorded investment	$ 0	$ 82,882	$ 0	$ 110,443
Post-modification outstanding recorded investment	$ 0	$ 100,205	$ 0	$ 119,890
Residential Mortgage | Residential mortgage | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	6	10	10	25
Pre-modification outstanding recorded investment	$ 521	$ 1,438	$ 780	$ 4,733
Post-modification outstanding recorded investment	$ 492	$ 1,438	$ 730	$ 4,692
Consumer and Other Loans | Home Equity | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	33	36	66	57
Pre-modification outstanding recorded investment	$ 4,820	$ 2,750	$ 10,851	$ 3,928
Post-modification outstanding recorded investment	$ 4,807	$ 2,750	$ 10,771	$ 3,916
Consumer and Other Loans | Consumer - indirect automobile | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	23	0	33	0
Pre-modification outstanding recorded investment	$ 224	$ 0	$ 360	$ 0
Post-modification outstanding recorded investment	$ 114	$ 0	$ 222	$ 0
Consumer and Other Loans | Consumer - other | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	30	61	50	85
Pre-modification outstanding recorded investment	$ 832	$ 946	$ 1,547	$ 1,386
Post-modification outstanding recorded investment	827	946	1,490	1,328
Extended maturities
Financing Receivable, Modifications
Post-modification outstanding recorded investment	8,488	54,758	15,014	57,533
Maturity and interest rate adjustment
Financing Receivable, Modifications
Post-modification outstanding recorded investment	3,886	30,781	6,502	31,048
Movement to or extension of interest-rate only payments
Financing Receivable, Modifications
Post-modification outstanding recorded investment	38	440	138	440
Interest rate adjustment
Financing Receivable, Modifications
Post-modification outstanding recorded investment	26	134	26	134
Forbearance
Financing Receivable, Modifications
Post-modification outstanding recorded investment	3,466	33,426	4,687	38,367
Other concession(s) (1)
Financing Receivable, Modifications
Post-modification outstanding recorded investment	$ 1,988	$ 15,985	$ 2,031	$ 37,761