Loans and Allowance for Credit Losses for Loans - Past Due, Non-Accrual and Current Loans by Loan Portfolio Class (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	$ 185,282	$ 93,058
Total Past Due Loans	284,287	161,284
Current Loans	31,932,825	22,908,325
Total Loans	32,217,112
Non-Accrual Loans Without Allowance for Credit Losses	48,151
PCI Loans		6,629,599
30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	69,712	46,593
60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	15,752	14,315
90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	13,541	7,318
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	106,693	68,636
Total Past Due Loans	124,445	86,549
Current Loans	6,737,263	4,057,434
Non-Accrual Loans Without Allowance for Credit Losses	4,075
PCI Loans		682,014
Commercial and industrial | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	6,393	11,700
Commercial and industrial | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	2,252	2,227
Commercial and industrial | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	9,107	3,986
Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	46,963	9,360
Total Past Due Loans	84,627	19,354
Current Loans	18,386,196	12,379,256
Total Loans	18,470,823
Non-Accrual Loans Without Allowance for Credit Losses	32,416
PCI Loans		5,245,149
Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	35,345	4,046
Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,326	5,369
Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	993	579
Commercial real estate | Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	46,879	9,004
Total Past Due Loans	84,228	16,169
Current Loans	16,640,770	10,886,724
Non-Accrual Loans Without Allowance for Credit Losses	32,416
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	35,030	2,560
Commercial real estate | Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,326	4,026
Commercial real estate | Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	993	579
Commercial real estate | Construction
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	84	356
Total Past Due Loans	399	3,185
Current Loans	1,745,426	1,492,532
Non-Accrual Loans Without Allowance for Credit Losses	0
PCI Loans		151,301
Commercial real estate | Construction | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	315	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	25,817	12,858
Total Past Due Loans	57,055	36,235
Current Loans	4,126,688	3,760,707
Non-Accrual Loans Without Allowance for Credit Losses	11,610
PCI Loans		580,169
Residential mortgage | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	17,717	17,143
Residential mortgage | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	10,351	4,192
Residential mortgage | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,170	2,042
Consumer loans:
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	5,809	2,204
Total Past Due Loans	18,160	19,146
Current Loans	2,682,678	2,710,928
Total Loans	2,700,838
Non-Accrual Loans Without Allowance for Credit Losses	50
PCI Loans		122,267
Consumer loans: | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	10,257	13,704
Consumer loans: | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,823	2,527
Consumer loans: | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	271	711
Consumer loans: | Home equity
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	4,936	1,646
Total Past Due Loans	6,381	2,777
Current Loans	425,172	373,243
Non-Accrual Loans Without Allowance for Credit Losses	50
PCI Loans		111,252
Consumer loans: | Home equity | 30-59 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	953	1,051
Consumer loans: | Home equity | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	492	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	338	334
Total Past Due Loans	9,746	14,078
Current Loans	1,346,209	1,437,274
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	8,056	11,482
Consumer loans: | Automobile | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,107	1,581
Consumer loans: | Automobile | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	245	681
Consumer loans: | Other consumer
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	535	224
Total Past Due Loans	2,033	2,291
Current Loans	911,297	900,411
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	1,248	1,171
Consumer loans: | Other consumer | 60-89 Days Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	224	866
Consumer loans: | Other consumer | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	$ 26	$ 30