Loans - Recorded Investment in Loan Classes Based on Payment Activity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Non-PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 5,023,485	$ 4,768,989
Non-PCI Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2,717,744	2,684,195
Non-PCI Loans | Consumer | Home equity
Financing Receivable, Recorded Investment [Line Items]
Total Loans	373,631	376,213
Non-PCI Loans | Consumer | Automobile
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,208,804	1,139,082
Non-PCI Loans | Consumer | Other consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	723,306	569,499
Non-PCI Loans | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	5,009,210	4,755,740
Non-PCI Loans | Performing Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2,705,339	2,672,120
Non-PCI Loans | Performing Loans | Consumer | Home equity
Financing Receivable, Recorded Investment [Line Items]
Total Loans	371,854	375,185
Non-PCI Loans | Performing Loans | Consumer | Automobile
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,208,731	1,138,936
Non-PCI Loans | Performing Loans | Consumer | Other consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	723,286	569,499
Non-PCI Loans | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	14,275	13,249
Non-PCI Loans | Non-Performing Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	12,405	12,075
Non-PCI Loans | Non-Performing Loans | Consumer | Home equity
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,777	1,028
Non-PCI Loans | Non-Performing Loans | Consumer | Automobile
Financing Receivable, Recorded Investment [Line Items]
Total Loans	73	146
Non-PCI Loans | Non-Performing Loans | Consumer | Other consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	20	0
PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,387,215	1,771,502
PCI Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	141,291	183,723
PCI Loans | Consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	77,497	100,583
PCI Loans | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total Loans	192,360	281,177
PCI Loans | Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans	934,926	1,091,339
PCI Loans | Commercial real estate | Construction
Financing Receivable, Recorded Investment [Line Items]
Total Loans	41,141	114,680
PCI Loans | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,349,127	1,744,491
PCI Loans | Performing Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	135,745	179,793
PCI Loans | Performing Loans | Consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	76,901	98,469
PCI Loans | Performing Loans | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total Loans	172,105	272,483
PCI Loans | Performing Loans | Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans	924,574	1,080,376
PCI Loans | Performing Loans | Commercial real estate | Construction
Financing Receivable, Recorded Investment [Line Items]
Total Loans	39,802	113,370
PCI Loans | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	38,088	27,011
PCI Loans | Non-Performing Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	5,546	3,930
PCI Loans | Non-Performing Loans | Consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	596	2,114
PCI Loans | Non-Performing Loans | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total Loans	20,255	8,694
PCI Loans | Non-Performing Loans | Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans	10,352	10,963
PCI Loans | Non-Performing Loans | Commercial real estate | Construction
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 1,339	$ 1,310