Loans - Past Due, Non-Accrual and Current Non-Covered Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	$ 101,049	$ 88,396
Total Past Due Loans	189,526	156,101
Current Non-PCI Loans	22,840,465	20,689,282
Total Non-PCI Loans	23,029,991	20,845,383
30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	64,100	51,751
60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	17,016	8,142
Accruing Loans 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due Loans	7,361	7,812
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	75,311	70,096
Total Past Due Loans	88,304	93,105
Current Non-PCI Loans	3,949,760	3,497,270
Total Non-PCI Loans	4,038,064	3,590,375
Commercial and industrial | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	5,702	13,085
Commercial and industrial | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,158	3,768
Commercial and industrial | Accruing Loans 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due Loans	4,133	6,156
Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	9,916	2,728
Total Past Due Loans	51,279	15,635
Current Non-PCI Loans	12,455,116	11,019,022
Total Non-PCI Loans	12,506,395	11,034,657
Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	32,374	12,350
Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	7,864	530
Commercial real estate | Accruing Loans 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,125	27
Commercial real estate | Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	9,560	2,372
Total Past Due Loans	32,271	12,450
Current Non-PCI Loans	11,089,054	9,899,859
Total Non-PCI Loans	11,121,325	9,912,309
Commercial real estate | Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	20,851	9,521
Commercial real estate | Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	735	530
Commercial real estate | Commercial real estate | Accruing Loans 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,125	27
Commercial real estate | Construction
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	356	356
Total Past Due Loans	19,008	3,185
Current Non-PCI Loans	1,366,062	1,119,163
Total Non-PCI Loans	1,385,070	1,122,348
Commercial real estate | Construction | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	11,523	2,829
Commercial real estate | Construction | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	7,129	0
Commercial real estate | Construction | Accruing Loans 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Residential mortgage
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	13,772	12,917
Total Past Due Loans	32,481	33,239
Current Non-PCI Loans	3,738,019	3,649,745
Total Non-PCI Loans	3,770,500	3,682,984
Residential mortgage | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	12,945	16,576
Residential mortgage | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	4,417	2,458
Residential mortgage | Accruing Loans 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,347	1,288
Consumer loans
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	2,050	2,655
Total Past Due Loans	17,462	14,122
Current Non-PCI Loans	2,697,570	2,523,245
Total Non-PCI Loans	2,715,032	2,537,367
Consumer loans | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	13,079	9,740
Consumer loans | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,577	1,386
Consumer loans | Accruing Loans 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due Loans	756	341
Consumer loans | Home equity
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	1,625	2,156
Total Past Due Loans	2,236	3,068
Current Non-PCI Loans	379,843	368,272
Total Non-PCI Loans	382,079	371,340
Consumer loans | Home equity | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	362	872
Consumer loans | Home equity | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	249	40
Consumer loans | Home equity | Accruing Loans 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Consumer loans | Automobile
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	207	80
Total Past Due Loans	11,193	9,660
Current Non-PCI Loans	1,425,120	1,309,546
Total Non-PCI Loans	1,436,313	1,319,206
Consumer loans | Automobile | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	9,118	7,973
Consumer loans | Automobile | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,220	1,299
Consumer loans | Automobile | Accruing Loans 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due Loans	648	308
Consumer loans | Other consumer
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	218	419
Total Past Due Loans	4,033	1,394
Current Non-PCI Loans	892,607	845,427
Total Non-PCI Loans	896,640	846,821
Consumer loans | Other consumer | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,599	895
Consumer loans | Other consumer | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	108	47
Consumer loans | Other consumer | Accruing Loans 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due Loans	$ 108	$ 33