Loans and Allowances for Credit Losses, By Aging Category (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	$ 112,058	$ 88,718
Current and Past Due Performing Loans	20,347,936	23,007,520
Total	20,347,936	23,007,520
30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	48,848	68,013
60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	23,803	38,782
90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	162,390	182,227
Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	20,112,895	22,718,498
Commercial [Member]
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	200	4,821
Current and Past Due Performing Loans	12,175,140	13,077,535
Total	12,175,140	13,077,535
Commercial [Member] | 30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	5,228	17,219
Commercial [Member] | 60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	3,469	10,504
Commercial [Member] | 90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	27,679	71,011
Commercial [Member] | Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	12,138,764	12,978,801
Commercial [Member] | Energy [Member]
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	0	0
Current and Past Due Performing Loans	2,814,059	3,469,194
Total	2,814,059	3,469,194
Commercial [Member] | Energy [Member] | 30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	0	12,735
Commercial [Member] | Energy [Member] | 60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	0	4,050
Commercial [Member] | Energy [Member] | 90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	10,571	41,414
Commercial [Member] | Energy [Member] | Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	2,803,488	3,410,995
Commercial [Member] | Healthcare [Member]
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	0	0
Current and Past Due Performing Loans	3,347,641	3,305,990
Total	3,347,641	3,305,990
Commercial [Member] | Healthcare [Member] | 30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	404	0
Commercial [Member] | Healthcare [Member] | 60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	0	0
Commercial [Member] | Healthcare [Member] | 90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	497	3,645
Commercial [Member] | Healthcare [Member] | Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	3,346,740	3,302,345
Commercial [Member] | Services [Member]
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	0	326
Current and Past Due Performing Loans	3,323,422	3,508,583
Total	3,323,422	3,508,583
Commercial [Member] | Services [Member] | 30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	3,092	3,278
Commercial [Member] | Services [Member] | 60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	1,648	177
Commercial [Member] | Services [Member] | 90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	9,024	15,705
Commercial [Member] | Services [Member] | Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	3,309,658	3,489,423
Commercial [Member] | General business [Member]
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	200	4,495
Current and Past Due Performing Loans	2,690,018	2,793,768
Total	2,690,018	2,793,768
Commercial [Member] | General business [Member] | 30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	1,732	1,206
Commercial [Member] | General business [Member] | 60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	1,821	6,277
Commercial [Member] | General business [Member] | 90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	7,587	10,247
Commercial [Member] | General business [Member] | Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	2,678,878	2,776,038
Commercial real estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	0	5,126
Current and Past Due Performing Loans	4,116,892	4,698,538
Total	4,116,892	4,698,538
Commercial real estate [Member] | 30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	1,505	276
Commercial real estate [Member] | 60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	0	5,310
Commercial real estate [Member] | 90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	9,596	20,673
Commercial real estate [Member] | Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	4,105,791	4,672,279
Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	0	0
Current and Past Due Performing Loans	536,052	1,682,310
Total	536,052	1,682,310
Paycheck Protection Program [Member] | 30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	0	0
Paycheck Protection Program [Member] | 60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	0	0
Paycheck Protection Program [Member] | 90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	0	0
Paycheck Protection Program [Member] | Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	536,052	1,682,310
Loans to individuals [Member]
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	111,858	78,771
Current and Past Due Performing Loans	3,519,852	3,549,137
Total	3,519,852	3,549,137
Loans to individuals [Member] | 30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	42,115	50,518
Loans to individuals [Member] | 60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	20,334	22,968
Loans to individuals [Member] | 90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	125,115	90,543
Loans to individuals [Member] | Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	3,332,288	3,385,108
Loans to individuals [Member] | Residential mortgage [Member]
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	23	181
Current and Past Due Performing Loans	1,747,243	1,863,003
Total	1,747,243	1,863,003
Loans to individuals [Member] | Residential mortgage [Member] | 30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	6,147	5,812
Loans to individuals [Member] | Residential mortgage [Member] | 60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	2,269	837
Loans to individuals [Member] | Residential mortgage [Member] | 90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	6,885	7,050
Loans to individuals [Member] | Residential mortgage [Member] | Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	1,731,942	1,849,304
Loans to individuals [Member] | Residential mortgage [Member] | US Government Agency Insured Loans [Member]
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	111,835	78,349
Current and Past Due Performing Loans	376,986	408,687
Total	376,986	408,687
Loans to individuals [Member] | Residential mortgage [Member] | US Government Agency Insured Loans [Member] | 30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	35,894	41,389
Loans to individuals [Member] | Residential mortgage [Member] | US Government Agency Insured Loans [Member] | 60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	18,028	22,041
Loans to individuals [Member] | Residential mortgage [Member] | US Government Agency Insured Loans [Member] | 90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	118,163	83,155
Loans to individuals [Member] | Residential mortgage [Member] | US Government Agency Insured Loans [Member] | Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	204,901	262,102
Loans to individuals [Member] | Personal [Member]
Financing Receivable, Past Due [Line Items]
Past Due 90 Days or More and Accruing	0	241
Current and Past Due Performing Loans	1,395,623	1,277,447
Total	1,395,623	1,277,447
Loans to individuals [Member] | Personal [Member] | 30 to 59 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	74	3,317
Loans to individuals [Member] | Personal [Member] | 60 to 89 Days [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	37	90
Loans to individuals [Member] | Personal [Member] | 90 Days or More [Member]
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	67	338
Loans to individuals [Member] | Personal [Member] | Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Current and Past Due Performing Loans	$ 1,395,445	$ 1,273,702