Allowance for Credit Losses - Summary of Allocation of Allowance for Loan Losses and Related Loans by Loan Portfolio Segment Disaggregated Based on Impairment Methodology (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 12,525	$ 23,525
Collectively evaluated for impairment	89,628	83,022
Loans acquired with discounts related to credit quality	200	7,070
Total	102,353	113,617
Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	145,008	191,348
Collectively evaluated for impairment	11,607,104	10,569,996
Loans acquired with discounts related to credit quality	1,721,801	806,268
Total	13,473,913	11,567,612
Commercial and Industrial [Member] | Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	4,929	11,032
Collectively evaluated for impairment	38,577	40,007
Loans acquired with discounts related to credit quality	170	512
Total	43,676	51,551
Commercial and Industrial [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	28,224	47,303
Collectively evaluated for impairment	1,931,703	1,772,833
Loans acquired with discounts related to credit quality	291,184	201,197
Total	2,251,111	2,021,333
Commercial real estate loans [Member] | Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	5,502	8,676
Collectively evaluated for impairment	37,338	27,235
Loans acquired with discounts related to credit quality	0	6,432
Total	42,840	42,343
Commercial real estate loans [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	91,298	114,638
Collectively evaluated for impairment	5,438,538	4,814,159
Loans acquired with discounts related to credit quality	1,164,179	543,603
Total	6,694,015	5,472,400
Residential mortgage [Member] | Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	1,629	3,735
Collectively evaluated for impairment	3,434	3,928
Loans acquired with discounts related to credit quality	30	123
Total	5,093	7,786
Residential mortgage [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	22,340	28,313
Collectively evaluated for impairment	2,396,704	2,456,926
Loans acquired with discounts related to credit quality	157,328	22,349
Total	2,576,372	2,507,588
Consumer [Member] | Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	465	82
Collectively evaluated for impairment	4,714	4,274
Loans acquired with discounts related to credit quality	0	3
Total	5,179	4,359
Consumer [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	3,146	1,094
Collectively evaluated for impairment	1,840,159	1,526,078
Loans acquired with discounts related to credit quality	109,110	39,119
Total	1,952,415	1,566,291
Unallocated [Member] | Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	5,565	7,578
Loans acquired with discounts related to credit quality	0	0
Total	5,565	7,578
Unallocated [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	0	0
Loans acquired with discounts related to credit quality	0	0
Total	$ 0	$ 0