Loans - Past Due, Non-Accrual and Current Non-Covered Loans by Loan Portfolio Class (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	$ 47,225	$ 37,508
Total Past Due Loans	127,706	94,221
Current Non-PCI Loans	16,816,659	15,370,380
Total Non-PCI Loans	16,944,365	15,464,601
30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	48,900	34,878
60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	28,491	18,363
Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,090	3,472
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	20,890	8,465
Total Past Due Loans	25,084	20,322
Current Non-PCI Loans	2,523,981	2,336,696
Total Non-PCI Loans	2,549,065	2,357,018
Commercial and industrial | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,650	6,705
Commercial and industrial | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	544	5,010
Commercial and industrial | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	142
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	12,060	15,794
Total Past Due Loans	55,104	37,987
Current Non-PCI Loans	9,316,711	8,300,607
Total Non-PCI Loans	9,371,815	8,338,594
Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	24,172	11,971
Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	18,845	8,642
Commercial real estate | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	27	1,580
Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	11,328	15,079
Total Past Due Loans	22,578	30,089
Current Non-PCI Loans	8,539,273	7,598,239
Total Non-PCI Loans	8,561,851	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	11,223	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	0	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	27	474
Commercial real estate | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	732	715
Total Past Due Loans	32,526	7,898
Current Non-PCI Loans	777,438	702,368
Total Non-PCI Loans	809,964	710,266
Commercial real estate | Construction | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	12,949	6,077
Commercial real estate | Construction | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	18,845	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	12,405	12,075
Total Past Due Loans	35,756	29,185
Current Non-PCI Loans	2,681,988	2,655,010
Total Non-PCI Loans	2,717,744	2,684,195
Residential mortgage | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	12,669	12,005
Residential mortgage | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	7,903	3,564
Residential mortgage | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,779	1,541
Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,870	1,174
Total Past Due Loans	11,762	6,727
Current Non-PCI Loans	2,293,979	2,078,067
Total Non-PCI Loans	2,305,741	2,084,794
Consumer loans | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	8,409	4,197
Consumer loans | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,199	1,147
Consumer loans | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	284	209
Consumer loans | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,777	1,028
Total Past Due Loans	2,880	2,372
Current Non-PCI Loans	370,751	373,841
Total Non-PCI Loans	373,631	376,213
Consumer loans | Home equity | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,009	929
Consumer loans | Home equity | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	94	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	73	146
Total Past Due Loans	7,038	4,249
Current Non-PCI Loans	1,201,766	1,134,833
Total Non-PCI Loans	1,208,804	1,139,082
Consumer loans | Automobile | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	5,707	3,192
Consumer loans | Automobile | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	987	723
Consumer loans | Automobile | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	271	188
Consumer loans | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	20	0
Total Past Due Loans	1,844	106
Current Non-PCI Loans	721,462	569,393
Total Non-PCI Loans	723,306	569,499
Consumer loans | Other consumer | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,693	76
Consumer loans | Other consumer | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	118	9
Consumer loans | Other consumer | Accruing Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	$ 13	$ 21