Loans - Past Due, Non-Accrual and Current Non-Covered Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	$ 96,515	$ 88,396
Total Past Due Loans	163,560	156,101
Current Non-PCI Loans	21,866,645	20,689,282
Total Non-PCI Loans	22,030,205	20,845,383
30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	51,384	51,751
60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	10,950	8,142
Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,711	7,812
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	76,216	70,096
Total Past Due Loans	97,768	93,105
Current Non-PCI Loans	3,821,126	3,497,270
Total Non-PCI Loans	3,918,894	3,590,375
Commercial and industrial | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	14,119	13,085
Commercial and industrial | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,135	3,768
Commercial and industrial | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,298	6,156
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	6,231	2,728
Total Past Due Loans	14,129	15,635
Current Non-PCI Loans	11,744,415	11,019,022
Total Non-PCI Loans	11,758,544	11,034,657
Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	6,202	12,350
Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,696	530
Commercial real estate | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	27
Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	6,231	2,372
Total Past Due Loans	12,787	12,450
Current Non-PCI Loans	10,457,340	9,899,859
Total Non-PCI Loans	10,470,127	9,912,309
Commercial real estate | Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	6,202	9,521
Commercial real estate | Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	354	530
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	0	27
Commercial real estate | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	0	356
Total Past Due Loans	1,342	3,185
Current Non-PCI Loans	1,287,075	1,119,163
Total Non-PCI Loans	1,288,417	1,122,348
Commercial real estate | Construction | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	2,829
Commercial real estate | Construction | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,342	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	0
Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	12,069	12,917
Total Past Due Loans	35,889	33,239
Current Non-PCI Loans	3,658,574	3,649,745
Total Non-PCI Loans	3,694,463	3,682,984
Residential mortgage | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	19,131	16,576
Residential mortgage | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,635	2,458
Residential mortgage | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,054	1,288
Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,999	2,655
Total Past Due Loans	15,774	14,122
Current Non-PCI Loans	2,642,530	2,523,245
Total Non-PCI Loans	2,658,304	2,537,367
Consumer loans | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	11,932	9,740
Consumer loans | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,484	1,386
Consumer loans | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	359	341
Consumer loans | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,630	2,156
Total Past Due Loans	2,385	3,068
Current Non-PCI Loans	383,602	368,272
Total Non-PCI Loans	385,987	371,340
Consumer loans | Home equity | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	673	872
Consumer loans | Home equity | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	82	40
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	148	80
Total Past Due Loans	8,622	9,660
Current Non-PCI Loans	1,353,521	1,309,546
Total Non-PCI Loans	1,362,143	1,319,206
Consumer loans | Automobile | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	7,221	7,973
Consumer loans | Automobile | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	925	1,299
Consumer loans | Automobile | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	328	308
Consumer loans | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	221	419
Total Past Due Loans	4,767	1,394
Current Non-PCI Loans	905,407	845,427
Total Non-PCI Loans	910,174	846,821
Consumer loans | Other consumer | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,038	895
Consumer loans | Other consumer | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	477	47
Consumer loans | Other consumer | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	$ 31	$ 33