Loans and Allowances for Credit Losses, By Aging Category (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing receivable, recorded investment, aging [Abstract]
Current	$ 22,207,452	$ 21,392,306
Nonaccrual	163,172	180,951
Total	22,463,970	21,750,987
Past Due 90 Days or More and Accruing	100,686	7,680
30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	49,503	47,084
60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	23,311	20,560
90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	183,704	110,086
Commercial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	14,714,100	13,899,429
Nonaccrual	118,624	115,416
Total	14,795,975	14,031,650
Past Due 90 Days or More and Accruing	788
Commercial [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	10,104	7,943
Commercial [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	4,960	1,608
Commercial [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	66,811	7,254
Commercial [Member] | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	4,058,605	3,881,244
Nonaccrual	96,448	91,722
Total	4,111,676	3,973,377
Past Due 90 Days or More and Accruing	0
Commercial [Member] | Energy [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	267	401
Commercial [Member] | Energy [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	10
Commercial [Member] | Energy [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	52,804	0
Commercial [Member] | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,160,196	3,027,329
Nonaccrual	4,070	4,480
Total	3,165,096	3,033,916
Past Due 90 Days or More and Accruing	714
Commercial [Member] | Healthcare [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	46	2,039
Commercial [Member] | Healthcare [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	111	0
Commercial [Member] | Healthcare [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	4,743	68
Commercial [Member] | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,941,795	3,105,621
Nonaccrual	8,425	7,483
Total	3,955,748	3,122,163
Past Due 90 Days or More and Accruing	74
Commercial [Member] | Services [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	7,173	1,737
Commercial [Member] | Services [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	198	523
Commercial [Member] | Services [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	6,582	6,799
Commercial [Member] | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		1,758,878
Nonaccrual		1,163
Total		1,760,866
Commercial [Member] | Wholesale/retail [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		712
Commercial [Member] | Wholesale/retail [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		113
Commercial [Member] | Wholesale/retail [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial [Member] | General business [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,553,504
Nonaccrual	9,681
Total	3,563,455
Past Due 90 Days or More and Accruing	0
Commercial [Member] | General business [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	2,618
Commercial [Member] | General business [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	4,651
Commercial [Member] | General business [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	2,682
Commercial [Member] | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		654,329
Nonaccrual		10,133
Total		665,449
Commercial [Member] | Manufacturing [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		410
Commercial [Member] | Manufacturing [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		190
Commercial [Member] | Manufacturing [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		387
Commercial [Member] | Public finance [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		707,638
Nonaccrual		0
Total		709,868
Commercial [Member] | Public finance [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		2,230
Commercial [Member] | Public finance [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial [Member] | Public finance [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial [Member] | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		764,390
Nonaccrual		435
Total		766,011
Commercial [Member] | Other commercial and industrial [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		414
Commercial [Member] | Other commercial and industrial [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		772
Commercial [Member] | Other commercial and industrial [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	4,433,887	4,400,576
Nonaccrual	8,545	27,626
Total	4,450,085	4,433,783
Past Due 90 Days or More and Accruing	2,796
Commercial real estate [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	4,523	4,920
Commercial real estate [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	340	250
Commercial real estate [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	11,335	411
Commercial real estate [Member] | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		147,379
Nonaccrual		350
Total		150,879
Commercial real estate [Member] | Residential construction and land development [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		3,093
Commercial real estate [Member] | Residential construction and land development [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Residential construction and land development [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		57
Commercial real estate [Member] | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		756,653
Nonaccrual		18,868
Total		775,521
Commercial real estate [Member] | Retail [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Retail [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Retail [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		928,379
Nonaccrual		0
Total		928,379
Commercial real estate [Member] | Office [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Office [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Office [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		1,258,704
Nonaccrual		6,858
Total		1,265,562
Commercial real estate [Member] | Multifamily [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Multifamily [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Multifamily [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		855,208
Nonaccrual		909
Total		856,117
Commercial real estate [Member] | Industrial [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Industrial [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Industrial [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Commercial real estate [Member] | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		454,253
Nonaccrual		641
Total		457,325
Commercial real estate [Member] | Other commercial real estate [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		1,827
Commercial real estate [Member] | Other commercial real estate [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		250
Commercial real estate [Member] | Other commercial real estate [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		354
Loans to individuals [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,059,465	1,895,939
Nonaccrual	36,003	37,622
Total	3,217,910	2,084,172
Past Due 90 Days or More and Accruing	97,102
Loans to individuals [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	34,876	29,557
Loans to individuals [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	18,011	18,648
Loans to individuals [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	105,558	102,406
Loans to individuals [Member] | Residential mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,828,421
Nonaccrual	30,721
Total	1,844,555	2,084,172
Past Due 90 Days or More and Accruing	119
Loans to individuals [Member] | Residential mortgage [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	8,759
Loans to individuals [Member] | Residential mortgage [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,717
Loans to individuals [Member] | Residential mortgage [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	5,658
Loans to individuals [Member] | Residential mortgage [Member] | US Government Agency Insured Loans [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	55,901
Nonaccrual	5,005
Total	197,889
Past Due 90 Days or More and Accruing	96,980
Loans to individuals [Member] | Residential mortgage [Member] | US Government Agency Insured Loans [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	25,948
Loans to individuals [Member] | Residential mortgage [Member] | US Government Agency Insured Loans [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	16,217
Loans to individuals [Member] | Residential mortgage [Member] | US Government Agency Insured Loans [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	99,823
Loans to individuals [Member] | Permanent Residential Mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		1,034,716
Nonaccrual		20,441
Total		1,057,321
Loans to individuals [Member] | Permanent Residential Mortgage [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		2,011
Loans to individuals [Member] | Permanent Residential Mortgage [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		153
Loans to individuals [Member] | Permanent Residential Mortgage [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Loans to individuals [Member] | Permanent Residential Mortgage [Member] | US Government Agency Insured Loans [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		46,898
Nonaccrual		6,100
Total		197,794
Loans to individuals [Member] | Permanent Residential Mortgage [Member] | US Government Agency Insured Loans [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		24,203
Loans to individuals [Member] | Permanent Residential Mortgage [Member] | US Government Agency Insured Loans [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		18,187
Loans to individuals [Member] | Permanent Residential Mortgage [Member] | US Government Agency Insured Loans [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		102,406
Loans to individuals [Member] | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Current		814,325
Nonaccrual		11,081
Total		829,057
Loans to individuals [Member] | Home equity [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		3,343
Loans to individuals [Member] | Home equity [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		308
Loans to individuals [Member] | Home equity [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due		0
Loans to individuals [Member] | Personal [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,175,143	1,196,362
Nonaccrual	277	287
Total	1,175,466	1,201,382
Past Due 90 Days or More and Accruing	3
Loans to individuals [Member] | Personal [Member] | 30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	169	4,664
Loans to individuals [Member] | Personal [Member] | 60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	77	54
Loans to individuals [Member] | Personal [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	$ 77	$ 15