Allowance for Credit Losses - Allocation of Allowance for Loan Losses and Related Loans by Segment (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	$ 22,552,578	$ 18,331,580
Allowance for loan losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	25,101	14,561
Collectively evaluated for impairment	107,761	106,295
Total Loans	132,862	120,856
Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	162,533	164,166
Collectively evaluated for impairment	17,474,212	16,780,199
Loans | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	4,915,833	1,387,215
Commercial and Industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	3,631,208	2,741,425
Commercial and Industrial | Allowance for loan losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	21,664	11,044
Collectively evaluated for impairment	44,882	46,188
Total Loans	66,546	57,232
Commercial and Industrial | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	89,668	85,499
Collectively evaluated for impairment	2,646,113	2,463,566
Commercial and Industrial | Loans | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	895,427	192,360
Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	13,078,936	10,347,882
Commercial Real Estate | Allowance for loan losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	2,672	2,735
Collectively evaluated for impairment	54,007	52,219
Total Loans	56,679	54,954
Commercial Real Estate | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	56,947	60,851
Collectively evaluated for impairment	9,698,719	9,310,964
Commercial Real Estate | Loans | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	3,323,270	976,067
Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	3,321,560	2,859,035
Residential Mortgage | Allowance for loan losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	705	718
Collectively evaluated for impairment	3,395	2,887
Total Loans	4,100	3,605
Residential Mortgage | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	14,030	14,056
Collectively evaluated for impairment	2,803,027	2,703,688
Residential Mortgage | Loans | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	504,503	141,291
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	2,520,874	2,383,238
Consumer | Allowance for loan losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	60	64
Collectively evaluated for impairment	5,477	5,001
Total Loans	5,537	5,065
Consumer | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	1,888	3,760
Collectively evaluated for impairment	2,326,353	2,301,981
Consumer | Loans | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	$ 192,633	$ 77,497