Loans and Allowance for Credit Losses for Loans - Past Due, Non-Accrual and Current Non-Covered Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	$ 191,109	$ 93,058
Total Past Due Loans	275,000	161,284
Current Loans	32,140,586	22,908,325
Total Loans	32,415,586
Non-Accrual Loans Without Allowance for Credit Losses	67,441
PCI Loans		6,629,599
30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	64,274	46,593
60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	9,676	14,315
90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	9,941	7,318
Commercial and Industrial
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	115,667	68,636
Total Past Due Loans	132,967	86,549
Current Loans	6,770,378	4,057,434
Total Loans	6,903,345
Non-Accrual Loans Without Allowance for Credit Losses	16,812
PCI Loans		682,014
Commercial and Industrial | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	6,587	11,700
Commercial and Industrial | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,954	2,227
Commercial and Industrial | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	6,759	3,986
Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	44,124	9,360
Total Past Due Loans	72,452	19,354
Current Loans	18,463,910	12,379,256
Total Loans	18,536,362
Non-Accrual Loans Without Allowance for Credit Losses	38,203
PCI Loans		5,245,149
Commercial real estate | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	26,180	4,046
Commercial real estate | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	610	5,369
Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,538	579
Commercial real estate | Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	41,627	9,004
Total Past Due Loans	69,813	16,169
Current Loans	16,745,774	10,886,724
Total Loans	16,815,587
Non-Accrual Loans Without Allowance for Credit Losses	35,798
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	26,038	2,560
Commercial real estate | Commercial real estate | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	610	4,026
Commercial real estate | Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,538	579
Commercial real estate | Construction
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	2,497	356
Total Past Due Loans	2,639	3,185
Current Loans	1,718,136	1,492,532
Total Loans	1,720,775
Non-Accrual Loans Without Allowance for Credit Losses	2,405
PCI Loans		151,301
Commercial real estate | Construction | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	142	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	23,877	12,858
Total Past Due Loans	51,056	36,235
Current Loans	4,233,539	3,760,707
Total Loans	4,284,595
Non-Accrual Loans Without Allowance for Credit Losses	12,356
PCI Loans		580,169
Residential mortgage | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	22,528	17,143
Residential mortgage | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,760	4,192
Residential mortgage | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	891	2,042
Consumer loans
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	7,441	2,204
Total Past Due Loans	18,525	19,146
Current Loans	2,672,759	2,710,928
Total Loans	2,691,284
Non-Accrual Loans Without Allowance for Credit Losses	70
PCI Loans		122,267
Consumer loans | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	8,979	13,704
Consumer loans | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,352	2,527
Consumer loans | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	753	711
Consumer loans | Home equity
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	6,969	1,646
Total Past Due Loans	8,549	2,777
Current Loans	448,534	373,243
Total Loans	457,083
Non-Accrual Loans Without Allowance for Credit Losses	70
PCI Loans		111,252
Consumer loans | Home equity | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,281	1,051
Consumer loans | Home equity | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	299	80
Consumer loans | Home equity | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Consumer loans | Automobile
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	472	334
Total Past Due Loans	6,615	14,078
Current Loans	1,335,044	1,437,274
Total Loans	1,341,659
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	4,797	11,482
Consumer loans | Automobile | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	808	1,581
Consumer loans | Automobile | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	538	681
Consumer loans | Other consumer
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	0	224
Total Past Due Loans	3,361	2,291
Current Loans	889,181	900,411
Total Loans	892,542
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	2,901	1,171
Consumer loans | Other consumer | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	245	866
Consumer loans | Other consumer | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	$ 215	$ 30