Loans and leases and the allowance for credit losses - Allocation of Allowance for Credit Losses on Basis of Company's Impairment Methodology (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ 69,651	$ 81,714
Allowance for credit losses	767,133	753,052
Allowance for credit losses	921,373	919,562	916,768	916,676
Purchased Impaired [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	8,453	9,142
Allocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	845,237	843,908
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	76,136	75,654	75,932	75,015
Commercial, Financial, Leasing, etc. [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	19,335	31,779
Allowance for credit losses	258,028	251,607
Commercial, Financial, Leasing, etc. [Member] | Purchased Impaired [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	3,706	4,652
Commercial, Financial, Leasing, etc. [Member] | Allocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	281,069	288,038
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	16,921	15,490
Allowance for credit losses	299,262	291,244
Commercial Real Estate [Member] | Purchased Impaired [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	1,192	1,193
Commercial Real Estate [Member] | Allocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	317,375	307,927
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	14,811	14,703
Allowance for credit losses	43,547	45,061
Residential Real Estate [Member] | Purchased Impaired [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	2,383	2,146
Residential Real Estate [Member] | Allocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	60,741	61,910
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	18,584	19,742
Allowance for credit losses	166,296	165,140
Consumer Loans [Member] | Purchased Impaired [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	1,172	1,151
Consumer Loans [Member] | Allocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ 186,052	$ 186,033