Loans - Past Due, Non-Accrual and Current Non-Covered Loans by Loan Portfolio Class (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	$ 88,396	$ 47,225
Total Past Due Loans	156,101	127,706
Current Non-PCI Loans	20,689,282	16,816,659
Total Non-PCI Loans	20,845,383	16,944,365
30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	51,751	48,900
60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	8,142	28,491
Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	7,812	3,090
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	70,096	20,890
Total Past Due Loans	93,105	25,084
Current Non-PCI Loans	3,497,270	2,523,981
Total Non-PCI Loans	3,590,375	2,549,065
Commercial and industrial | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	13,085	3,650
Commercial and industrial | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,768	544
Commercial and industrial | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	6,156	0
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	2,728	12,060
Total Past Due Loans	15,635	55,104
Current Non-PCI Loans	11,019,022	9,316,711
Total Non-PCI Loans	11,034,657	9,371,815
Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	12,350	24,172
Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	530	18,845
Commercial real estate | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	27	27
Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	2,372	11,328
Total Past Due Loans	12,450	22,578
Current Non-PCI Loans	9,899,859	8,539,273
Total Non-PCI Loans	9,912,309	8,561,851
Commercial real estate | Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	9,521	11,223
Commercial real estate | Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	530	0
Commercial real estate | Commercial real estate | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	27	27
Commercial real estate | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	356	732
Total Past Due Loans	3,185	32,526
Current Non-PCI Loans	1,119,163	777,438
Total Non-PCI Loans	1,122,348	809,964
Commercial real estate | Construction | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,829	12,949
Commercial real estate | Construction | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	18,845
Commercial real estate | Construction | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	0
Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	12,917	12,405
Total Past Due Loans	33,239	35,756
Current Non-PCI Loans	3,649,745	2,681,988
Total Non-PCI Loans	3,682,984	2,717,744
Residential mortgage | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	16,576	12,669
Residential mortgage | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,458	7,903
Residential mortgage | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,288	2,779
Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	2,655	1,870
Total Past Due Loans	14,122	11,762
Current Non-PCI Loans	2,523,245	2,293,979
Total Non-PCI Loans	2,537,367	2,305,741
Consumer loans | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	9,740	8,409
Consumer loans | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,386	1,199
Consumer loans | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	341	284
Consumer loans | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	2,156	1,777
Total Past Due Loans	3,068	2,880
Current Non-PCI Loans	368,272	370,751
Total Non-PCI Loans	371,340	373,631
Consumer loans | Home equity | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	872	1,009
Consumer loans | Home equity | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	40	94
Consumer loans | Home equity | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	0
Consumer loans | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	80	73
Total Past Due Loans	9,660	7,038
Current Non-PCI Loans	1,309,546	1,201,766
Total Non-PCI Loans	1,319,206	1,208,804
Consumer loans | Automobile | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	7,973	5,707
Consumer loans | Automobile | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,299	987
Consumer loans | Automobile | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	308	271
Consumer loans | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	419	20
Total Past Due Loans	1,394	1,844
Current Non-PCI Loans	845,427	721,462
Total Non-PCI Loans	846,821	723,306
Consumer loans | Other consumer | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	895	1,693
Consumer loans | Other consumer | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	47	118
Consumer loans | Other consumer | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	$ 33	$ 13