Loans - Past Due, Non-Accrual and Current Non-Covered Loans by Loan Portfolio Class (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due Loans	$ 20,198	$ 27,113
60-89 Days Past Due Loans	5,554	10,313
Accruing Loans 90 Days Or More Past Due	5,478	9,491
Non-Accrual Loans	55,757	95,063
Total Past Due Loans	86,987	141,980
Current Non-PCI Loans	11,665,125	10,619,364
Total Non-PCI Loans	11,752,112	10,761,344
Commercial real estate loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due Loans	9,386	10,328
60-89 Days Past Due Loans	113	7,019
Accruing Loans 90 Days Or More Past Due	4,037	7,591
Non-Accrual Loans	27,321	52,050
Total Past Due Loans	40,857	76,988
Current Non-PCI Loans	5,488,979	4,851,809
Total Non-PCI Loans	5,529,836	4,928,797
Consumer loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due Loans	2,982	3,792
60-89 Days Past Due Loans	764	784
Accruing Loans 90 Days Or More Past Due	152	118
Non-Accrual Loans	2,209	2,035
Total Past Due Loans	6,107	6,729
Current Non-PCI Loans	1,837,198	1,520,443
Total Non-PCI Loans	1,843,305	1,527,172
Commercial and industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due Loans	1,630	6,398
60-89 Days Past Due Loans	1,102	571
Accruing Loans 90 Days Or More Past Due	226	233
Non-Accrual Loans	8,467	21,029
Total Past Due Loans	11,425	28,231
Current Non-PCI Loans	1,948,502	1,791,905
Total Non-PCI Loans	1,959,927	1,820,136
Commercial real estate [Member] | Commercial real estate loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due Loans	8,938	9,142
60-89 Days Past Due Loans	113	2,442
Accruing Loans 90 Days Or More Past Due	49	7,591
Non-Accrual Loans	22,098	43,934
Total Past Due Loans	31,198	63,109
Current Non-PCI Loans	5,022,544	4,458,811
Total Non-PCI Loans	5,053,742	4,521,920
Construction [Member] | Commercial real estate loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due Loans	448	1,186
60-89 Days Past Due Loans	0	4,577
Accruing Loans 90 Days Or More Past Due	3,988	0
Non-Accrual Loans	5,223	8,116
Total Past Due Loans	9,659	13,879
Current Non-PCI Loans	466,435	392,998
Total Non-PCI Loans	476,094	406,877
Residential mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due Loans	6,200	6,595
60-89 Days Past Due Loans	3,575	1,939
Accruing Loans 90 Days Or More Past Due	1,063	1,549
Non-Accrual Loans	17,760	19,949
Total Past Due Loans	28,598	30,032
Current Non-PCI Loans	2,390,446	2,455,207
Total Non-PCI Loans	2,419,044	2,485,239
Home equity [Member] | Consumer loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due Loans	761	495
60-89 Days Past Due Loans	282	241
Accruing Loans 90 Days Or More Past Due	0	0
Non-Accrual Loans	2,022	1,866
Total Past Due Loans	3,065	2,602
Current Non-PCI Loans	397,071	408,273
Total Non-PCI Loans	400,136	410,875
Automobile [Member] | Consumer loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due Loans	1,902	2,957
60-89 Days Past Due Loans	391	489
Accruing Loans 90 Days Or More Past Due	126	85
Non-Accrual Loans	90	169
Total Past Due Loans	2,509	3,700
Current Non-PCI Loans	1,142,271	897,699
Total Non-PCI Loans	1,144,780	901,399
Other Consumer [Member] | Consumer loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due Loans	319	340
60-89 Days Past Due Loans	91	54
Accruing Loans 90 Days Or More Past Due	26	33
Non-Accrual Loans	97	0
Total Past Due Loans	533	427
Current Non-PCI Loans	297,856	214,471
Total Non-PCI Loans	$ 298,389	$ 214,898