Loans - Past Due, Non-Accrual and Current Non-Covered Loans by Loan Portfolio Class (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 97,159	$ 86,987
Accruing Loans 90 Days Or More Past Due	3,826	5,478
Non-Accrual Loans	57,544	55,757
Current Non-PCI Loans	11,988,120	11,665,125
Past Due Non Accrual And Current Non Pci Loans	12,085,279	11,752,112
Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	46,021	40,857
Accruing Loans 90 Days Or More Past Due	2,792	4,037
Non-Accrual Loans	29,994	27,321
Current Non-PCI Loans	5,580,250	5,488,979
Past Due Non Accrual And Current Non Pci Loans	5,626,271	5,529,836
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	6,112	6,107
Accruing Loans 90 Days Or More Past Due	262	152
Non-Accrual Loans	2,138	2,209
Current Non-PCI Loans	1,862,795	1,837,198
Past Due Non Accrual And Current Non Pci Loans	1,868,907	1,843,305
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	13,055	11,425
Accruing Loans 90 Days Or More Past Due	208	226
Non-Accrual Loans	8,285	8,467
Current Non-PCI Loans	2,081,219	1,948,502
Past Due Non Accrual And Current Non Pci Loans	2,094,274	1,959,927
Commercial Portfolio Segment [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	34,300	31,198
Accruing Loans 90 Days Or More Past Due	2,792	49
Non-Accrual Loans	24,850	22,098
Current Non-PCI Loans	5,108,288	5,022,544
Past Due Non Accrual And Current Non Pci Loans	5,142,588	5,053,742
Construction [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	11,721	9,659
Accruing Loans 90 Days Or More Past Due	0	3,988
Non-Accrual Loans	5,144	5,223
Current Non-PCI Loans	471,962	466,435
Past Due Non Accrual And Current Non Pci Loans	483,683	476,094
Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	31,971	28,598
Accruing Loans 90 Days Or More Past Due	564	1,063
Non-Accrual Loans	17,127	17,760
Current Non-PCI Loans	2,463,856	2,390,446
Past Due Non Accrual And Current Non Pci Loans	2,495,827	2,419,044
Home Equity [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	3,592	3,065
Accruing Loans 90 Days Or More Past Due	0	0
Non-Accrual Loans	2,042	2,022
Current Non-PCI Loans	391,294	397,071
Past Due Non Accrual And Current Non Pci Loans	394,886	400,136
Automobile [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,260	2,509
Accruing Loans 90 Days Or More Past Due	251	126
Non-Accrual Loans	0	90
Current Non-PCI Loans	1,160,659	1,142,271
Past Due Non Accrual And Current Non Pci Loans	1,162,919	1,144,780
Other Consumer [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	260	533
Accruing Loans 90 Days Or More Past Due	11	26
Non-Accrual Loans	96	97
Current Non-PCI Loans	310,842	297,856
Past Due Non Accrual And Current Non Pci Loans	311,102	298,389
Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	31,197	20,198
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	11,352	9,386
Financing Receivables, 30 to 59 Days Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,875	2,982
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	4,472	1,630
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	4,775	8,938
Financing Receivables, 30 to 59 Days Past Due [Member] | Construction [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	6,577	448
Financing Receivables, 30 to 59 Days Past Due [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	12,498	6,200
Financing Receivables, 30 to 59 Days Past Due [Member] | Home Equity [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,170	761
Financing Receivables, 30 to 59 Days Past Due [Member] | Automobile [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,594	1,902
Financing Receivables, 30 to 59 Days Past Due [Member] | Other Consumer [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	111	319
Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	4,592	5,554
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,883	113
Financing Receivables, 60 to 89 Days Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	837	764
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	90	1,102
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,883	113
Financing Receivables, 60 to 89 Days Past Due [Member] | Construction [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Financing Receivables, 60 to 89 Days Past Due [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,782	3,575
Financing Receivables, 60 to 89 Days Past Due [Member] | Home Equity [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	380	282
Financing Receivables, 60 to 89 Days Past Due [Member] | Automobile [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	415	391
Financing Receivables, 60 to 89 Days Past Due [Member] | Other Consumer [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 42	$ 91