Loans - Recorded Investment in Loan Classes Based on Payment Activity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Non-PCI Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 4,364,734	$ 4,262,349
Non-PCI Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,495,827	2,419,044
Non-PCI Loans [Member] | Home Equity [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	394,886	400,136
Non-PCI Loans [Member] | Automobile [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,162,919	1,144,780
Non-PCI Loans [Member] | Other Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	311,102	298,389
PCI Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,649,182	1,721,801
PCI Loans [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	273,653	291,184
PCI Loans [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,063,285	1,107,139
PCI Loans [Member] | Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	58,331	57,040
PCI Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	152,184	157,328
PCI Loans [Member] | Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	101,729	109,110
Performing Loans [Member] | Non-PCI Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	4,345,469	4,242,380
Performing Loans [Member] | Non-PCI Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,478,700	2,401,284
Performing Loans [Member] | Non-PCI Loans [Member] | Home Equity [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	392,844	398,114
Performing Loans [Member] | Non-PCI Loans [Member] | Automobile [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,162,919	1,144,690
Performing Loans [Member] | Non-PCI Loans [Member] | Other Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	311,006	298,292
Performing Loans [Member] | PCI Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,604,262	1,674,088
Performing Loans [Member] | PCI Loans [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	253,446	272,027
Performing Loans [Member] | PCI Loans [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,054,600	1,091,784
Performing Loans [Member] | PCI Loans [Member] | Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	57,204	52,802
Performing Loans [Member] | PCI Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	148,972	153,789
Performing Loans [Member] | PCI Loans [Member] | Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	90,040	103,686
Non-Performing Loans [Member] | Non-PCI Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	19,265	19,969
Non-Performing Loans [Member] | Non-PCI Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	17,127	17,760
Non-Performing Loans [Member] | Non-PCI Loans [Member] | Home Equity [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,042	2,022
Non-Performing Loans [Member] | Non-PCI Loans [Member] | Automobile [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	90
Non-Performing Loans [Member] | Non-PCI Loans [Member] | Other Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	96	97
Non-Performing Loans [Member] | PCI Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	44,920	47,713
Non-Performing Loans [Member] | PCI Loans [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	20,207	19,157
Non-Performing Loans [Member] | PCI Loans [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	8,685	15,355
Non-Performing Loans [Member] | PCI Loans [Member] | Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,127	4,238
Non-Performing Loans [Member] | PCI Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	3,212	3,539
Non-Performing Loans [Member] | PCI Loans [Member] | Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 11,689	$ 5,424