Loans and Allowance for Credit Losses for Loans - Past Due, Non-Accrual and Current Loans by Loan Portfolio Class (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	$ 204,010	$ 185,282
Total Past Due Loans	256,793	284,287
Current Loans	32,429,623	31,932,825
Total	32,686,416	32,217,112
Non-Accrual Loans Without Allowance for Credit Losses	69,590	48,151
30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	36,902	69,712
60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	10,477	15,752
90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	5,404	13,541
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	108,988	106,693
Total Past Due Loans	117,034	124,445
Current Loans	7,031,610	6,737,263
Total	7,148,644	6,861,708
Non-Accrual Loans Without Allowance for Credit Losses	10,618	4,075
Commercial and industrial | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,763	6,393
Commercial and industrial | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,768	2,252
Commercial and industrial | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	2,515	9,107
Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	54,075	46,963
Total Past Due Loans	71,185	84,627
Current Loans	18,638,773	18,386,196
Total	18,709,958	18,470,823
Non-Accrual Loans Without Allowance for Credit Losses	41,706	32,416
Commercial real estate | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	11,655	35,345
Commercial real estate | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	5,455	1,326
Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	993
Commercial real estate | Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	54,004	46,879
Total Past Due Loans	71,114	84,228
Current Loans	16,852,513	16,640,770
Total	16,923,627	16,724,998
Non-Accrual Loans Without Allowance for Credit Losses	41,706	32,416
Commercial real estate | Commercial real estate | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	11,655	35,030
Commercial real estate | Commercial real estate | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	5,455	1,326
Commercial real estate | Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	993
Commercial real estate | Construction
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	71	84
Total Past Due Loans	71	399
Current Loans	1,786,260	1,745,426
Total	1,786,331	1,745,825
Non-Accrual Loans Without Allowance for Credit Losses	0	0
Commercial real estate | Construction | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	315
Commercial real estate | Construction | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
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	33,655	25,817
Total Past Due Loans	54,364	57,055
Current Loans	4,006,128	4,126,688
Total	4,060,492	4,183,743
Non-Accrual Loans Without Allowance for Credit Losses	17,218	11,610
Residential mortgage | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	16,004	17,717
Residential mortgage | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	2,233	10,351
Residential mortgage | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	2,472	3,170
Total	23,180	18,270
Consumer loans
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	7,292	5,809
Total Past Due Loans	14,210	18,160
Current Loans	2,753,112	2,682,678
Total	2,767,322	2,700,838
Non-Accrual Loans Without Allowance for Credit Losses	48	50
Consumer loans | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	5,480	10,257
Consumer loans | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,021	1,823
Consumer loans | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	417	271
Consumer loans | Home equity
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	6,411	4,936
Total Past Due Loans	7,295	6,381
Current Loans	402,281	425,172
Total	409,576	431,553
Non-Accrual Loans Without Allowance for Credit Losses	48	50
Consumer loans | Home equity | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	737	953
Consumer loans | Home equity | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	147	492
Consumer loans | Home equity | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Total	1,479	1,216
Consumer loans | Automobile
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	368	338
Total Past Due Loans	5,949	9,746
Current Loans	1,438,934	1,346,209
Total	1,444,883	1,355,955
Non-Accrual Loans Without Allowance for Credit Losses	0	0
Consumer loans | Automobile | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	4,421	8,056
Consumer loans | Automobile | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	770	1,107
Consumer loans | Automobile | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	390	245
Total	769	652
Consumer loans | Other consumer
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	513	535
Total Past Due Loans	966	2,033
Current Loans	911,897	911,297
Total	912,863	913,330
Non-Accrual Loans Without Allowance for Credit Losses	0	0
Consumer loans | Other consumer | 30-59  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	322	1,248
Consumer loans | Other consumer | 60-89  Days  Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	104	224
Consumer loans | Other consumer | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	27	26
Total	$ 409	$ 435