Loans - Schedule of Modified TDRs (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) SecurityLoan	Sep. 30, 2016 USD ($) SecurityLoan	Sep. 30, 2017 USD ($) SecurityLoan	Sep. 30, 2016 USD ($) SecurityLoan
Financing Receivable, Modifications
Post-modification outstanding recorded investment	$ 24,586	$ 75,068	$ 51,517	$ 218,596
Troubled Debt Restructurings
Financing Receivable, Modifications
Post-modification outstanding recorded investment	$ 24,586	$ 75,068	$ 51,500	$ 218,600
TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	107	141	293	377
Pre-modification outstanding recorded investment	$ 23,821	$ 87,482	$ 49,445	$ 224,148
Post-modification outstanding recorded investment			$ 51,517	$ 218,596
Commercial Loans | Commercial real estate- construction | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	2	0	2	1
Pre-modification outstanding recorded investment	$ 1,166	$ 0	$ 1,166	$ 28
Post-modification outstanding recorded investment	$ 1,164	$ 0	$ 1,164	$ 25
Commercial Loans | Commercial real estate- owner-occupied | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	3	5	5	9
Pre-modification outstanding recorded investment	$ 717	$ 1,254	$ 2,447	$ 8,069
Post-modification outstanding recorded investment	$ 713	$ 1,254	$ 2,411	$ 8,043
Commercial Loans | Commercial real estate- non-owner-occupied | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	10	6	21	13
Pre-modification outstanding recorded investment	$ 5,306	$ 9,512	$ 9,645	$ 14,668
Post-modification outstanding recorded investment	$ 5,298	$ 3,688	$ 11,505	$ 8,373
Commercial Loans | Commercial and industrial | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	11	24	44	49
Pre-modification outstanding recorded investment	$ 11,650	$ 20,207	$ 18,743	$ 44,144
Post-modification outstanding recorded investment	$ 12,502	$ 20,175	$ 19,399	$ 43,083
Commercial Loans | Energy-related | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	0	11	0	28
Pre-modification outstanding recorded investment	$ 0	$ 51,979	$ 0	$ 143,012
Post-modification outstanding recorded investment	$ 0	$ 45,682	$ 0	$ 145,298
Residential Mortgage | Residential mortgage | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	7	7	16	31
Pre-modification outstanding recorded investment	$ 409	$ 937	$ 1,126	$ 4,784
Post-modification outstanding recorded investment	$ 392	$ 910	$ 1,030	$ 4,659
Consumer and Other Loans | Home Equity | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	38	37	99	104
Pre-modification outstanding recorded investment	$ 3,495	$ 2,570	$ 13,573	$ 7,071
Post-modification outstanding recorded investment	$ 3,479	$ 2,371	$ 13,471	$ 6,805
Consumer and Other Loans | Consumer - indirect automobile | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	11	19	36	63
Pre-modification outstanding recorded investment	$ 126	$ 273	$ 398	$ 691
Post-modification outstanding recorded investment	$ 114	$ 244	$ 279	$ 693
Consumer and Other Loans | Consumer - other | TDRs Occurring during the Period | Non-Covered TDRs
Financing Receivable, Modifications
Number of loans | SecurityLoan	25	32	70	79
Pre-modification outstanding recorded investment	$ 952	$ 750	$ 2,347	$ 1,681
Post-modification outstanding recorded investment	924	744	2,258	1,617
Extended maturities
Financing Receivable, Modifications
Post-modification outstanding recorded investment	3,184	19,620	18,177	76,059
Maturity and interest rate adjustment
Financing Receivable, Modifications
Post-modification outstanding recorded investment	204	1,958	5,151	26,372
Movement to or extension of interest-rate only payments
Financing Receivable, Modifications
Post-modification outstanding recorded investment	3,560	1,256	3,692	1,689
Interest rate adjustment
Financing Receivable, Modifications
Post-modification outstanding recorded investment	0	0	25	133
Forbearance
Financing Receivable, Modifications
Post-modification outstanding recorded investment	841	51,285	5,528	75,953
Other concession(s) (1)
Financing Receivable, Modifications
Post-modification outstanding recorded investment	$ 16,797	$ 949	$ 18,944	$ 38,390