Loans and Allowance for Credit Losses for Loans - Past Due, Non-Accrual and Current Non-Covered Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	$ 210,585	$ 93,058
Total Past Due Loans	303,718	161,284
Current Loans	32,010,893	22,908,325
Total Loans	32,314,611
Non-Accrual Loans Without Allowance for Credit Losses	70,669
PCI Loans		6,629,599
30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	68,343	46,593
60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	13,676	14,315
90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	11,114	7,318
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	130,876	68,636
Total Past Due Loans	146,480	86,549
Current Loans	6,738,209	4,057,434
Total Loans	6,884,689
Non-Accrual Loans Without Allowance for Credit Losses	13,501
PCI Loans		682,014
Commercial and industrial | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	6,206	11,700
Commercial and industrial | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	4,178	2,227
Commercial and industrial | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	5,220	3,986
Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	46,986	9,360
Total Past Due Loans	62,441	19,354
Current Loans	18,230,788	12,379,256
Total Loans	18,293,229
Non-Accrual Loans Without Allowance for Credit Losses	43,306
PCI Loans		5,245,149
Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	13,912	4,046
Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,543	5,369
Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	579
Commercial real estate | Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	43,678	9,004
Total Past Due Loans	59,133	16,169
Current Loans	16,512,744	10,886,724
Total Loans	16,571,877
Non-Accrual Loans Without Allowance for Credit Losses	40,476
PCI Loans		5,093,848
Commercial real estate | Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	13,912	2,560
Commercial real estate | Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,543	4,026
Commercial real estate | Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	579
Commercial real estate | Construction
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	3,308	356
Total Past Due Loans	3,308	3,185
Current Loans	1,718,044	1,492,532
Total Loans	1,721,352
Non-Accrual Loans Without Allowance for Credit Losses	2,830
PCI Loans		151,301
Commercial real estate | Construction | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	1,486
Commercial real estate | Construction | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	1,343
Commercial real estate | Construction | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Residential mortgage
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	25,776	12,858
Total Past Due Loans	69,020	36,235
Current Loans	4,336,127	3,760,707
Total Loans	4,405,147
Non-Accrual Loans Without Allowance for Credit Losses	13,717
PCI Loans		580,169
Residential mortgage | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	35,263	17,143
Residential mortgage | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	4,169	4,192
Residential mortgage | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,812	2,042
Consumer loans
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	6,947	2,204
Total Past Due Loans	25,777	19,146
Current Loans	2,705,769	2,710,928
Total Loans	2,731,546
Non-Accrual Loans Without Allowance for Credit Losses	145
PCI Loans		122,267
Consumer loans | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	12,962	13,704
Consumer loans | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,786	2,527
Consumer loans | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	2,082	711
Consumer loans | Home equity
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	5,326	1,646
Total Past Due Loans	9,789	2,777
Current Loans	461,326	373,243
Total Loans	471,115
Non-Accrual Loans Without Allowance for Credit Losses	145
PCI Loans		111,252
Consumer loans | Home equity | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,236	1,051
Consumer loans | Home equity | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	864	80
Consumer loans | Home equity | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	363	0
Consumer loans | Automobile
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	1,621	334
Total Past Due Loans	13,219	14,078
Current Loans	1,356,270	1,437,274
Total Loans	1,369,489
Non-Accrual Loans Without Allowance for Credit Losses	0
PCI Loans		271
Consumer loans | Automobile | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	8,254	11,482
Consumer loans | Automobile | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	2,171	1,581
Consumer loans | Automobile | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,173	681
Consumer loans | Other consumer
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	0	224
Total Past Due Loans	2,769	2,291
Current Loans	888,173	900,411
Total Loans	890,942
Non-Accrual Loans Without Allowance for Credit Losses	0
PCI Loans		10,744
Consumer loans | Other consumer | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,472	1,171
Consumer loans | Other consumer | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	751	866
Consumer loans | Other consumer | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	$ 546	$ 30