Loans - Recorded Investment in Loan Classes Based on Payment Activity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Non-PCI Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 4,262,349	$ 4,012,411
Non-PCI Loans [Member] | Residential mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2,419,044	2,485,239
Non-PCI Loans [Member] | Home equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	400,136	410,875
Non-PCI Loans [Member] | Automobile loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,144,780	901,399
Non-PCI Loans [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	298,389	214,898
PCI Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,721,801	806,268
PCI Loans [Member] | Commercial and industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	291,184	201,197
PCI Loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,107,139	521,249
PCI Loans [Member] | Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	57,040	22,354
PCI Loans [Member] | Residential mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	157,328	22,349
PCI Loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	109,110	39,119
Performing Loans [Member] | Non-PCI Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	4,242,380	3,990,427
Performing Loans [Member] | Non-PCI Loans [Member] | Residential mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2,401,284	2,465,290
Performing Loans [Member] | Non-PCI Loans [Member] | Home equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	398,114	409,009
Performing Loans [Member] | Non-PCI Loans [Member] | Automobile loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,144,690	901,230
Performing Loans [Member] | Non-PCI Loans [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	298,292	214,898
Performing Loans [Member] | PCI Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,674,088	759,521
Performing Loans [Member] | PCI Loans [Member] | Commercial and industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	272,027	185,185
Performing Loans [Member] | PCI Loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,091,784	498,184
Performing Loans [Member] | PCI Loans [Member] | Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	52,802	16,791
Performing Loans [Member] | PCI Loans [Member] | Residential mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	153,789	21,381
Performing Loans [Member] | PCI Loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	103,686	37,980
Non-Performing Loans [Member] | Non-PCI Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	19,969	21,984
Non-Performing Loans [Member] | Non-PCI Loans [Member] | Residential mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	17,760	19,949
Non-Performing Loans [Member] | Non-PCI Loans [Member] | Home equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2,022	1,866
Non-Performing Loans [Member] | Non-PCI Loans [Member] | Automobile loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	90	169
Non-Performing Loans [Member] | Non-PCI Loans [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	97	0
Non-Performing Loans [Member] | PCI Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	47,713	46,747
Non-Performing Loans [Member] | PCI Loans [Member] | Commercial and industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	19,157	16,012
Non-Performing Loans [Member] | PCI Loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	15,355	23,065
Non-Performing Loans [Member] | PCI Loans [Member] | Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	4,238	5,563
Non-Performing Loans [Member] | PCI Loans [Member] | Residential mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	3,539	968
Non-Performing Loans [Member] | PCI Loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 5,424	$ 1,139