Loans - Past Due, Non-Accrual and Current Non-Covered Loans by Loan Portfolio Class (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	$ 62,620	$ 62,100
Total Past Due Loans	99,109	88,240
Current Non-PCI Loans	13,921,457	13,714,396
Total Non-PCI Loans	14,020,566	13,802,636
30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	28,938	18,509
60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,520	5,575
Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,031	2,056
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	11,484	10,913
Total Past Due Loans	20,713	15,570
Current Non-PCI Loans	2,156,229	2,140,979
Total Non-PCI Loans	2,176,942	2,156,549
Commercial and industrial | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	8,395	3,920
Commercial and industrial | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	613	524
Commercial and industrial | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	221	213
Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	32,582	31,051
Total Past Due Loans	35,548	38,541
Current Non-PCI Loans	6,902,985	6,638,685
Total Non-PCI Loans	6,938,533	6,677,226
Commercial real estate loans | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	26,604	24,888
Total Past Due Loans	28,244	27,703
Current Non-PCI Loans	6,276,730	6,041,829
Total Non-PCI Loans	6,304,974	6,069,532
Commercial real estate loans | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	5,978	6,163
Total Past Due Loans	7,304	10,838
Current Non-PCI Loans	626,255	596,856
Total Non-PCI Loans	633,559	607,694
Commercial real estate loans | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,715	4,560
Commercial real estate loans | 30-59 Days Past Due Loans | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,389	2,684
Commercial real estate loans | 30-59 Days Past Due Loans | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,326	1,876
Commercial real estate loans | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	120	2,799
Commercial real estate loans | 60-89 Days Past Due Loans | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	120	0
Commercial real estate loans | 60-89 Days Past Due Loans | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	2,799
Commercial real estate loans | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	131	131
Commercial real estate loans | Accruing Loans 90 Days or More Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	131	131
Commercial real estate loans | Accruing Loans 90 Days or More Past Due | Construction
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	16,747	17,930
Total Past Due Loans	37,044	27,741
Current Non-PCI Loans	2,855,599	2,884,338
Total Non-PCI Loans	2,892,643	2,912,079
Residential mortgage | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	14,628	6,681
Residential mortgage | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,056	1,626
Residential mortgage | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,613	1,504
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,807	2,206
Total Past Due Loans	5,804	6,388
Current Non-PCI Loans	2,006,644	2,050,394
Total Non-PCI Loans	2,012,448	2,056,782
Consumer | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,685	2,088
Total Past Due Loans	3,247	3,507
Current Non-PCI Loans	377,212	388,302
Total Non-PCI Loans	380,459	391,809
Consumer | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	122	118
Total Past Due Loans	1,897	2,742
Current Non-PCI Loans	1,185,845	1,236,084
Total Non-PCI Loans	1,187,742	1,238,826
Consumer | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	0	0
Total Past Due Loans	660	139
Current Non-PCI Loans	443,587	426,008
Total Non-PCI Loans	444,247	426,147
Consumer | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,200	3,348
Consumer | 30-59 Days Past Due Loans | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,262	1,308
Consumer | 30-59 Days Past Due Loans | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,336	1,969
Consumer | 30-59 Days Past Due Loans | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	602	71
Consumer | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	731	626
Consumer | 60-89 Days Past Due Loans | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	300	111
Consumer | 60-89 Days Past Due Loans | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	374	491
Consumer | 60-89 Days Past Due Loans | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	57	24
Consumer | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	66	208
Consumer | Accruing Loans 90 Days or More Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	0
Consumer | Accruing Loans 90 Days or More Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	65	164
Consumer | Accruing Loans 90 Days or More Past Due | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	$ 1	$ 44