Loans - Past Due, Non-Accrual and Current Non-Covered Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	$ 41,787	$ 37,508
Total Past Due Loans	71,933	94,221
Current Non-PCI Loans	16,657,674	15,370,380
Total Non-PCI Loans	16,729,607	15,464,601
30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	19,088	34,878
60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	8,810	18,363
Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,248	3,472
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	11,983	8,465
Total Past Due Loans	16,337	20,322
Current Non-PCI Loans	2,488,318	2,336,696
Total Non-PCI Loans	2,504,655	2,357,018
Commercial and industrial | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,186	6,705
Commercial and industrial | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,043	5,010
Commercial and industrial | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	125	142
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	14,986	15,794
Total Past Due Loans	23,274	37,987
Current Non-PCI Loans	9,195,241	8,300,607
Total Non-PCI Loans	9,218,515	8,338,594
Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,755	11,971
Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,144	8,642
Commercial real estate | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	389	1,580
Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	13,870	15,079
Total Past Due Loans	19,640	30,089
Current Non-PCI Loans	8,340,193	7,598,239
Total Non-PCI Loans	8,359,833	7,628,328
Commercial real estate | Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,755	5,894
Commercial real estate | Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	626	8,642
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	389	474
Commercial real estate | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,116	715
Total Past Due Loans	3,634	7,898
Current Non-PCI Loans	855,048	702,368
Total Non-PCI Loans	858,682	710,266
Commercial real estate | Construction | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	6,077
Commercial real estate | Construction | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,518	0
Commercial real estate | Construction | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	1,106
Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	12,974	12,075
Total Past Due Loans	23,953	29,185
Current Non-PCI Loans	2,767,826	2,655,010
Total Non-PCI Loans	2,791,779	2,684,195
Residential mortgage | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	7,942	12,005
Residential mortgage | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,604	3,564
Residential mortgage | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,433	1,541
Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,844	1,174
Total Past Due Loans	8,369	6,727
Current Non-PCI Loans	2,206,289	2,078,067
Total Non-PCI Loans	2,214,658	2,084,794
Consumer loans | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	5,205	4,197
Consumer loans | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,019	1,147
Consumer loans | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	301	209
Consumer loans | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,766	1,028
Total Past Due Loans	2,789	2,372
Current Non-PCI Loans	368,341	373,841
Total Non-PCI Loans	371,130	376,213
Consumer loans | Home equity | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	591	929
Consumer loans | Home equity | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	432	415
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	78	146
Total Past Due Loans	5,030	4,249
Current Non-PCI Loans	1,166,549	1,134,833
Total Non-PCI Loans	1,171,579	1,139,082
Consumer loans | Automobile | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,089	3,192
Consumer loans | Automobile | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	566	723
Consumer loans | Automobile | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	297	188
Consumer loans | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	0	0
Total Past Due Loans	550	106
Current Non-PCI Loans	671,399	569,393
Total Non-PCI Loans	671,949	569,499
Consumer loans | Other consumer | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	525	76
Consumer loans | Other consumer | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	21	9
Consumer loans | Other consumer | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	$ 4	$ 21