Loans - Past Due, Non-Accrual and Current Non-Covered Loans by Loan Portfolio Class (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans 90 Days Or More Past Due	$ 4,103	$ 5,478
Non-Accrual Loans	59,224	55,757
Past Due Loans	88,283	86,987
Current Non-PCI Loans	13,450,743	11,665,125
Total Non-PCI Loans	13,539,026	11,752,112
Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans 90 Days Or More Past Due	245	4,037
Non-Accrual Loans	28,088	27,321
Past Due Loans	37,405	40,857
Current Non-PCI Loans	6,493,898	5,488,979
Total Non-PCI Loans	6,531,303	5,529,836
Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans 90 Days Or More Past Due	166	152
Non-Accrual Loans	1,634	2,209
Past Due Loans	6,013	6,107
Current Non-PCI Loans	1,985,670	1,837,198
Total Non-PCI Loans	1,991,683	1,843,305
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans 90 Days Or More Past Due	224	226
Non-Accrual Loans	12,845	8,467
Past Due Loans	17,146	11,425
Current Non-PCI Loans	2,135,492	1,948,502
Total Non-PCI Loans	2,152,638	1,959,927
Commercial real estate | Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans 90 Days Or More Past Due	245	49
Non-Accrual Loans	22,129	22,098
Past Due Loans	26,739	31,198
Current Non-PCI Loans	5,983,021	5,022,544
Total Non-PCI Loans	6,009,760	5,053,742
Construction | Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans 90 Days Or More Past Due	0	3,988
Non-Accrual Loans	5,959	5,223
Past Due Loans	10,666	9,659
Current Non-PCI Loans	510,877	466,435
Total Non-PCI Loans	521,543	476,094
Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans 90 Days Or More Past Due	3,468	1,063
Non-Accrual Loans	16,657	17,760
Past Due Loans	27,719	28,598
Current Non-PCI Loans	2,835,683	2,390,446
Total Non-PCI Loans	2,863,402	2,419,044
Home equity | Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans 90 Days Or More Past Due	0	0
Non-Accrual Loans	1,576	2,022
Past Due Loans	3,079	3,065
Current Non-PCI Loans	389,579	397,071
Total Non-PCI Loans	392,658	400,136
Automobile | Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans 90 Days Or More Past Due	164	126
Non-Accrual Loans	0	90
Past Due Loans	2,548	2,509
Current Non-PCI Loans	1,217,186	1,142,271
Total Non-PCI Loans	1,219,734	1,144,780
Other consumer | Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans 90 Days Or More Past Due	2	26
Non-Accrual Loans	58	97
Past Due Loans	386	533
Current Non-PCI Loans	378,905	297,856
Total Non-PCI Loans	379,291	298,389
30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	18,846	20,198
30-59 Days Past Due Loans | Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	7,657	9,386
30-59 Days Past Due Loans | Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,491	2,982
30-59 Days Past Due Loans | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,081	1,630
30-59 Days Past Due Loans | Commercial real estate | Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,950	8,938
30-59 Days Past Due Loans | Construction | Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,707	448
30-59 Days Past Due Loans | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	5,617	6,200
30-59 Days Past Due Loans | Home equity | Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,285	761
30-59 Days Past Due Loans | Automobile | Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,904	1,902
30-59 Days Past Due Loans | Other consumer | Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	302	319
60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	6,110	5,554
60-89 Days Past Due Loans | Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,415	113
60-89 Days Past Due Loans | Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	722	764
60-89 Days Past Due Loans | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,996	1,102
60-89 Days Past Due Loans | Commercial real estate | Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,415	113
60-89 Days Past Due Loans | Construction | Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	0
60-89 Days Past Due Loans | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,977	3,575
60-89 Days Past Due Loans | Home equity | Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	218	282
60-89 Days Past Due Loans | Automobile | Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	480	391
60-89 Days Past Due Loans | Other consumer | Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	$ 24	$ 91