Loans - Recorded Investment in Loan Classes Based on Payment Activity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Non-PCI Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 4,855,085	$ 4,262,349
Non-PCI Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,863,402	2,419,044
Non-PCI Loans | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	392,658	400,136
Non-PCI Loans | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,219,734	1,144,780
Non-PCI Loans | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	379,291	298,389
PCI Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,477,788	1,721,801
PCI Loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	247,980	291,184
PCI Loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	950,917	1,107,139
PCI Loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	48,110	57,040
PCI Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	135,860	157,328
PCI Loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	94,921	109,110
Performing Loans | Non-PCI Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	4,836,794	4,242,380
Performing Loans | Non-PCI Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,846,745	2,401,284
Performing Loans | Non-PCI Loans | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	391,082	398,114
Performing Loans | Non-PCI Loans | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,219,734	1,144,690
Performing Loans | Non-PCI Loans | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	379,233	298,292
Performing Loans | PCI Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,455,560	1,674,088
Performing Loans | PCI Loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	244,445	272,027
Performing Loans | PCI Loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	940,668	1,091,784
Performing Loans | PCI Loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	47,216	52,802
Performing Loans | PCI Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	132,649	153,789
Performing Loans | PCI Loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	90,582	103,686
Non-Performing Loans | Non-PCI Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	18,291	19,969
Non-Performing Loans | Non-PCI Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	16,657	17,760
Non-Performing Loans | Non-PCI Loans | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,576	2,022
Non-Performing Loans | Non-PCI Loans | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	90
Non-Performing Loans | Non-PCI Loans | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	58	97
Non-Performing Loans | PCI Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	22,228	47,713
Non-Performing Loans | PCI Loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	3,535	19,157
Non-Performing Loans | PCI Loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	10,249	15,355
Non-Performing Loans | PCI Loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	894	4,238
Non-Performing Loans | PCI Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	3,211	3,539
Non-Performing Loans | PCI Loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 4,339	$ 5,424