Loans - Recorded Investment Based on Payment Activity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Non-PCI Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 5,145,298	$ 5,023,485
Non-PCI Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,817,057	2,717,744
Non-PCI Loans | Consumer | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	372,348	373,631
Non-PCI Loans | Consumer | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,222,253	1,208,804
Non-PCI Loans | Consumer | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	733,640	723,306
Non-PCI Loans | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	5,119,500	5,009,210
Non-PCI Loans | Performing Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,794,363	2,705,339
Non-PCI Loans | Performing Loans | Consumer | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	369,458	371,854
Non-PCI Loans | Performing Loans | Consumer | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	1,222,165	1,208,731
Non-PCI Loans | Performing Loans | Consumer | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	733,514	723,286
Non-PCI Loans | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	25,798	14,275
Non-PCI Loans | Non-Performing Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	22,694	12,405
Non-PCI Loans | Non-Performing Loans | Consumer | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,890	1,777
Non-PCI Loans | Non-Performing Loans | Consumer | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	88	73
Non-PCI Loans | Non-Performing Loans | Consumer | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	126	20
PCI Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	4,915,833	1,387,215
PCI Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	504,503	141,291
PCI Loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	192,633	77,497
PCI Loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	895,427	192,360
PCI Loans | Commercial real estate | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,901,527	934,926
PCI Loans | Commercial real estate | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	421,743	41,141
PCI Loans | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	4,852,976	1,349,127
PCI Loans | Performing Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	498,036	135,745
PCI Loans | Performing Loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	190,560	76,901
PCI Loans | Performing Loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	867,135	172,105
PCI Loans | Performing Loans | Commercial real estate | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,877,254	924,574
PCI Loans | Performing Loans | Commercial real estate | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	419,991	39,802
PCI Loans | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	62,857	38,088
PCI Loans | Non-Performing Loans | Residential mortgage
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	6,467	5,546
PCI Loans | Non-Performing Loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,073	596
PCI Loans | Non-Performing Loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	28,292	20,255
PCI Loans | Non-Performing Loans | Commercial real estate | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	24,273	10,352
PCI Loans | Non-Performing Loans | Commercial real estate | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 1,752	$ 1,339