Loans and Allowance for Credit Losses for Loans - Past Due, Non-Accrual and Current Loans by Loan Portfolio Class (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Loans	$ 32,606,814	$ 32,217,112
Non-Accrual Loans	251,831	185,282
Non-Accrual Loans Without Allowance for Credit Losses	87,065	48,151
Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	307,045	284,287
30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	40,389	69,712
60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	9,473	15,752
90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	5,352	13,541
Current Loans
Financing Receivable, Past Due [Line Items]
Loans	32,299,769	31,932,825
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans	5,635,260	6,861,708
Non-Accrual Loans	100,614	106,693
Non-Accrual Loans Without Allowance for Credit Losses	10,407	4,075
Commercial and industrial | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	106,359	124,445
Commercial and industrial | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	2,677	6,393
Commercial and industrial | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	985	2,252
Commercial and industrial | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	2,083	9,107
Commercial and industrial | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	5,528,901	6,737,263
Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans	19,716,650	18,470,823
Non-Accrual Loans	113,496	46,963
Non-Accrual Loans Without Allowance for Credit Losses	55,621	32,416
Commercial real estate | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	144,291	84,627
Commercial real estate | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	22,956	35,345
Commercial real estate | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	5,897	1,326
Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	1,942	993
Commercial real estate | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	19,572,359	18,386,196
Commercial real estate | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans	17,912,070	16,724,998
Non-Accrual Loans	95,843	46,879
Non-Accrual Loans Without Allowance for Credit Losses	55,621	32,416
Commercial real estate | Commercial real estate | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	126,638	84,228
Commercial real estate | Commercial real estate | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	22,956	35,030
Commercial real estate | Commercial real estate | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	5,897	1,326
Commercial real estate | Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	1,942	993
Commercial real estate | Commercial real estate | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	17,785,432	16,640,770
Commercial real estate | Construction
Financing Receivable, Past Due [Line Items]
Loans	1,804,580	1,745,825
Non-Accrual Loans	17,653	84
Non-Accrual Loans Without Allowance for Credit Losses	0	0
Commercial real estate | Construction | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	17,653	399
Commercial real estate | Construction | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	0	315
Commercial real estate | Construction | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	0	0
Commercial real estate | Construction | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	0	0
Commercial real estate | Construction | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	1,786,927	1,745,426
Residential mortgage
Financing Receivable, Past Due [Line Items]
Loans	4,332,422	4,183,743
Non-Accrual Loans	33,648	25,817
Non-Accrual Loans Without Allowance for Credit Losses	21,032	11,610
Residential mortgage | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	44,917	57,055
Residential mortgage | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	9,293	17,717
Residential mortgage | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	974	10,351
Residential mortgage | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	1,002	3,170
Residential mortgage | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	4,287,505	4,126,688
Consumer
Financing Receivable, Past Due [Line Items]
Loans	2,922,482	2,700,838
Non-Accrual Loans	4,073	5,809
Non-Accrual Loans Without Allowance for Credit Losses	5	50
Consumer | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	11,478	18,160
Consumer | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	5,463	10,257
Consumer | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	1,617	1,823
Consumer | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	325	271
Consumer | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	2,911,004	2,682,678
Consumer | Home equity
Financing Receivable, Past Due [Line Items]
Loans	402,658	431,553
Non-Accrual Loans	3,541	4,936
Non-Accrual Loans Without Allowance for Credit Losses	5	50
Consumer | Home equity | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	4,372	6,381
Consumer | Home equity | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	606	953
Consumer | Home equity | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	225	492
Consumer | Home equity | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	0	0
Consumer | Home equity | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	398,286	425,172
Consumer | Automobile
Financing Receivable, Past Due [Line Items]
Loans	1,563,698	1,355,955
Non-Accrual Loans	420	338
Non-Accrual Loans Without Allowance for Credit Losses	0	0
Consumer | Automobile | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	5,426	9,746
Consumer | Automobile | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	4,218	8,056
Consumer | Automobile | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	555	1,107
Consumer | Automobile | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	233	245
Consumer | Automobile | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	1,558,272	1,346,209
Consumer | Other consumer
Financing Receivable, Past Due [Line Items]
Loans	956,126	913,330
Non-Accrual Loans	112	535
Non-Accrual Loans Without Allowance for Credit Losses	0	0
Consumer | Other consumer | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	1,680	2,033
Consumer | Other consumer | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	639	1,248
Consumer | Other consumer | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	837	224
Consumer | Other consumer | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	92	26
Consumer | Other consumer | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	$ 954,446	$ 911,297