Loans and Allowance for Credit Losses for Loans - Past Due, Non-Accrual and Current Loans by Loan Portfolio Class (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Loans	$ 32,457,454	$ 32,217,112
Non-Accrual Loans	220,012	185,282
Non-Accrual Loans Without Allowance for Credit Losses	63,521	48,151
Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	300,247	284,287
30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	59,112	69,712
60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	15,405	15,752
90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	5,718	13,541
Current Loans
Financing Receivable, Past Due [Line Items]
Loans	32,157,207	31,932,825
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans	6,084,455	6,861,708
Non-Accrual Loans	102,594	106,693
Non-Accrual Loans Without Allowance for Credit Losses	9,550	4,075
Commercial and industrial | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	110,173	124,445
Commercial and industrial | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	3,867	6,393
Commercial and industrial | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	1,361	2,252
Commercial and industrial | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	2,351	9,107
Commercial and industrial | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	5,974,282	6,737,263
Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans	19,264,980	18,470,823
Non-Accrual Loans	76,553	46,963
Non-Accrual Loans Without Allowance for Credit Losses	37,116	32,416
Commercial real estate | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	130,476	84,627
Commercial real estate | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	40,524	35,345
Commercial real estate | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	11,451	1,326
Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	1,948	993
Commercial real estate | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	19,134,504	18,386,196
Commercial real estate | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans	17,512,142	16,724,998
Non-Accrual Loans	58,893	46,879
Non-Accrual Loans Without Allowance for Credit Losses	37,116	32,416
Commercial real estate | Commercial real estate | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	112,816	84,228
Commercial real estate | Commercial real estate | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	40,524	35,030
Commercial real estate | Commercial real estate | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	11,451	1,326
Commercial real estate | Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	1,948	993
Commercial real estate | Commercial real estate | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	17,399,326	16,640,770
Commercial real estate | Construction
Financing Receivable, Past Due [Line Items]
Loans	1,752,838	1,745,825
Non-Accrual Loans	17,660	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	17,660	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	1,735,178	1,745,426
Residential mortgage
Financing Receivable, Past Due [Line Items]
Loans	4,226,975	4,183,743
Non-Accrual Loans	35,941	25,817
Non-Accrual Loans Without Allowance for Credit Losses	16,807	11,610
Residential mortgage | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	46,984	57,055
Residential mortgage | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	8,479	17,717
Residential mortgage | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	1,608	10,351
Residential mortgage | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	956	3,170
Residential mortgage | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	4,179,991	4,126,688
Consumer loans
Financing Receivable, Past Due [Line Items]
Loans	2,881,044	2,700,838
Non-Accrual Loans	4,924	5,809
Non-Accrual Loans Without Allowance for Credit Losses	48	50
Consumer loans | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	12,614	18,160
Consumer loans | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	6,242	10,257
Consumer loans | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	985	1,823
Consumer loans | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	463	271
Consumer loans | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	2,868,430	2,682,678
Consumer loans | Home equity
Financing Receivable, Past Due [Line Items]
Loans	410,856	431,553
Non-Accrual Loans	4,431	4,936
Non-Accrual Loans Without Allowance for Credit Losses	48	50
Consumer loans | Home equity | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	5,348	6,381
Consumer loans | Home equity | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	592	953
Consumer loans | Home equity | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	325	492
Consumer loans | Home equity | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	0	0
Consumer loans | Home equity | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	405,508	425,172
Consumer loans | Automobile
Financing Receivable, Past Due [Line Items]
Loans	1,531,262	1,355,955
Non-Accrual Loans	389	338
Non-Accrual Loans Without Allowance for Credit Losses	0	0
Consumer loans | Automobile | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	6,987	9,746
Consumer loans | Automobile | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	5,521	8,056
Consumer loans | Automobile | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	636	1,107
Consumer loans | Automobile | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	441	245
Consumer loans | Automobile | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	1,524,275	1,346,209
Consumer loans | Other consumer
Financing Receivable, Past Due [Line Items]
Loans	938,926	913,330
Non-Accrual Loans	104	535
Non-Accrual Loans Without Allowance for Credit Losses	0	0
Consumer loans | Other consumer | Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Loans	279	2,033
Consumer loans | Other consumer | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	129	1,248
Consumer loans | Other consumer | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	24	224
Consumer loans | Other consumer | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Loans	22	26
Consumer loans | Other consumer | Current Loans
Financing Receivable, Past Due [Line Items]
Loans	$ 938,647	$ 911,297