Loans - Recorded Investment in Loan Classes Based on Payment Activity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Non-PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 6,220,351	$ 5,023,485
Non-PCI Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	3,682,984	2,717,744
Non-PCI Loans | Consumer | Home equity
Financing Receivable, Recorded Investment [Line Items]
Total Loans	371,340	373,631
Non-PCI Loans | Consumer | Automobile
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,319,206	1,208,804
Non-PCI Loans | Consumer | Other consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	846,821	723,306
Non-PCI Loans | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	6,204,779	5,009,210
Non-PCI Loans | Performing Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	3,670,067	2,705,339
Non-PCI Loans | Performing Loans | Consumer | Home equity
Financing Receivable, Recorded Investment [Line Items]
Total Loans	369,184	371,854
Non-PCI Loans | Performing Loans | Consumer | Automobile
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,319,126	1,208,731
Non-PCI Loans | Performing Loans | Consumer | Other consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	846,402	723,286
Non-PCI Loans | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	15,572	14,275
Non-PCI Loans | Non-Performing Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	12,917	12,405
Non-PCI Loans | Non-Performing Loans | Consumer | Home equity
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2,156	1,777
Non-PCI Loans | Non-Performing Loans | Consumer | Automobile
Financing Receivable, Recorded Investment [Line Items]
Total Loans	80	73
Non-PCI Loans | Non-Performing Loans | Consumer | Other consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	419	20
PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	4,190,086	1,387,215
PCI Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	428,416	141,291
PCI Loans | Consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	160,263	77,497
PCI Loans | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total Loans	740,657	192,360
PCI Loans | Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2,494,966	934,926
PCI Loans | Commercial real estate | Construction
Financing Receivable, Recorded Investment [Line Items]
Total Loans	365,784	41,141
PCI Loans | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	4,133,961	1,349,127
PCI Loans | Performing Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	421,609	135,745
PCI Loans | Performing Loans | Consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	158,502	76,901
PCI Loans | Performing Loans | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total Loans	710,045	172,105
PCI Loans | Performing Loans | Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2,478,990	924,574
PCI Loans | Performing Loans | Commercial real estate | Construction
Financing Receivable, Recorded Investment [Line Items]
Total Loans	364,815	39,802
PCI Loans | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Total Loans	56,125	38,088
PCI Loans | Non-Performing Loans | Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Total Loans	6,807	5,546
PCI Loans | Non-Performing Loans | Consumer
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,761	596
PCI Loans | Non-Performing Loans | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total Loans	30,612	20,255
PCI Loans | Non-Performing Loans | Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans	15,976	10,352
PCI Loans | Non-Performing Loans | Commercial real estate | Construction
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 969	$ 1,339