Allowance for Credit Losses - Allocation of Allowance for Loan Losses and Related Loans by Segment (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	$ 18,201,462	$ 17,236,103
Allowance for loan losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	10,550	10,531
Collectively evaluated for impairment	108,416	103,888
Total Loans	118,966	114,419
Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	150,418	114,819
Collectively evaluated for impairment	16,579,189	15,349,782
Loans | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	1,471,855	1,771,502
Commercial and Industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	2,706,912	2,638,195
Commercial and Industrial | Allowance for loan losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	7,104	5,864
Collectively evaluated for impairment	47,585	44,956
Total Loans	54,689	50,820
Commercial and Industrial | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	68,944	30,640
Collectively evaluated for impairment	2,435,711	2,326,378
Commercial and Industrial | Loans | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	202,257	281,177
Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	10,254,708	9,544,613
Commercial Real Estate | Allowance for loan losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	2,644	3,872
Collectively evaluated for impairment	52,655	51,979
Total Loans	55,299	55,851
Commercial Real Estate | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	63,912	62,289
Collectively evaluated for impairment	9,154,603	8,276,305
Commercial Real Estate | Loans | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	1,036,193	1,206,019
Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	2,941,435	2,867,918
Residential Mortgage | Allowance for loan losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	733	725
Collectively evaluated for impairment	3,159	2,977
Total Loans	3,892	3,702
Residential Mortgage | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	14,313	18,356
Collectively evaluated for impairment	2,777,466	2,665,839
Residential Mortgage | Loans | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	149,656	183,723
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	2,298,407	2,185,377
Consumer | Allowance for loan losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	69	70
Collectively evaluated for impairment	5,017	3,976
Total Loans	5,086	4,046
Consumer | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	3,249	3,534
Collectively evaluated for impairment	2,211,409	2,081,260
Consumer | Loans | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	$ 83,749	$ 100,583