Recorded Investment in Loan Classes Based on Payment Activity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 11,148,938	$ 9,799,641
Total
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	3,876,680	3,664,318
Residential Mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,482,363	2,285,590
Home Equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	445,402	469,604
Automobile Loan
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	789,248	772,490
Other Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	159,667	136,634
Performing Financing Receivable | Total
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	3,841,285	3,628,762
Performing Financing Receivable | Residential Mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,450,799	2,253,944
Performing Financing Receivable | Home Equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	442,574	466,904
Performing Financing Receivable | Automobile Loan
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	788,904	772,029
Performing Financing Receivable | Other Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	159,008	135,885
Nonperforming Financing Receivable | Total
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	35,395	35,556
Nonperforming Financing Receivable | Residential Mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	31,564	31,646
Nonperforming Financing Receivable | Home Equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,828	2,700
Nonperforming Financing Receivable | Automobile Loan
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	344	461
Nonperforming Financing Receivable | Other Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 659	$ 749