Loans - Past Due, Non-Accrual and Current Non-Covered Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	$ 38,288	$ 37,508
Total Past Due Loans	106,430	94,221
Current Non-PCI Loans	15,688,367	15,370,380
Total Non-PCI Loans	15,794,797	15,464,601
30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	60,404	34,878
60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	5,664	18,363
Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,074	3,472
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	8,676	8,465
Total Past Due Loans	39,156	20,322
Current Non-PCI Loans	2,358,998	2,336,696
Total Non-PCI Loans	2,398,154	2,357,018
Commercial and industrial | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	29,734	6,705
Commercial and industrial | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	341	5,010
Commercial and industrial | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	405	142
Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	16,567	15,794
Total Past Due Loans	36,323	37,987
Current Non-PCI Loans	8,673,169	8,300,607
Total Non-PCI Loans	8,709,492	8,338,594
Commercial real estate loans | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	19,397	11,971
Commercial real estate loans | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	359	8,642
Commercial real estate loans | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	1,580
Commercial real estate loans | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	15,106	15,079
Total Past Due Loans	27,102	30,089
Current Non-PCI Loans	7,924,127	7,598,239
Total Non-PCI Loans	7,951,229	7,628,328
Commercial real estate loans | Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	11,637	5,894
Commercial real estate loans | Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	359	8,642
Commercial real estate loans | Commercial real estate | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	474
Commercial real estate loans | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,461	715
Total Past Due Loans	9,221	7,898
Current Non-PCI Loans	749,042	702,368
Total Non-PCI Loans	758,263	710,266
Commercial real estate loans | Construction | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	7,760	6,077
Commercial real estate loans | Construction | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	0
Commercial real estate loans | 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	11,650	12,075
Total Past Due Loans	24,715	29,185
Current Non-PCI Loans	2,549,631	2,655,010
Total Non-PCI Loans	2,574,346	2,684,195
Residential mortgage | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	7,533	12,005
Residential mortgage | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,177	3,564
Residential mortgage | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,355	1,541
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,395	1,174
Total Past Due Loans	6,236	6,727
Current Non-PCI Loans	2,106,569	2,078,067
Total Non-PCI Loans	2,112,805	2,084,794
Consumer | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,740	4,197
Consumer | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	787	1,147
Consumer | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	314	209
Consumer | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,285	1,028
Total Past Due Loans	2,307	2,372
Current Non-PCI Loans	366,925	373,841
Total Non-PCI Loans	369,232	376,213
Consumer | Home equity | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	725	929
Consumer | Home equity | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	297	415
Consumer | Home equity | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	0
Consumer | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	110	146
Total Past Due Loans	3,377	4,249
Current Non-PCI Loans	1,146,541	1,134,833
Total Non-PCI Loans	1,149,918	1,139,082
Consumer | Automobile | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,508	3,192
Consumer | Automobile | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	471	723
Consumer | Automobile | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	288	188
Consumer | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	0	0
Total Past Due Loans	552	106
Current Non-PCI Loans	593,103	569,393
Total Non-PCI Loans	593,655	569,499
Consumer | Other consumer | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	507	76
Consumer | Other consumer | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	19	9
Consumer | Other consumer | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	$ 26	$ 21