Loans and Allowances for Credit Losses, By Aging Category (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financing receivable, recorded investment, aging [Abstract]
Current	$ 16,597,641	$ 15,626,933
Nonaccrual	230,984	147,128
Total	16,989,660	15,941,154
Commercial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	10,205,838	10,165,995
Nonaccrual	178,953	76,424
Total	10,390,824	10,252,531
Commercial [Member] | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,364,890	3,033,504
Nonaccrual	132,499	61,189
Total	2,497,868	3,097,328
Commercial [Member] | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,099,605	2,769,895
Nonaccrual	8,173	10,290
Total	3,108,990	2,784,276
Commercial [Member] | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,561,650	1,418,396
Nonaccrual	11,407	2,919
Total	1,576,818	1,422,064
Commercial [Member] | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	509,662	556,398
Nonaccrual	4,931	331
Total	514,975	556,729
Commercial [Member] | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,201,050	1,879,873
Nonaccrual	825	1,072
Total	2,201,916	1,883,380
Commercial [Member] | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	468,981	507,929
Nonaccrual	21,118	623
Total	490,257	508,754
Commercial Real Estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,803,519	3,241,541
Nonaccrual	5,521	9,001
Total	3,809,046	3,259,033
Commercial Real Estate [Member] | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	132,100	156,017
Nonaccrual	3,433	4,409
Total	135,533	160,426
Commercial Real Estate [Member] | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	761,562	795,180
Nonaccrual	326	1,319
Total	761,888	796,499
Commercial Real Estate [Member] | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	798,462	637,056
Nonaccrual	426	651
Total	798,888	637,707
Commercial Real Estate [Member] | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	903,234	742,697
Nonaccrual	38	274
Total	903,272	751,085
Commercial Real Estate [Member] | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	871,673	563,093
Nonaccrual	76	76
Total	871,749	563,169
Commercial Real Estate [Member] | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	336,488	347,498
Nonaccrual	1,222	2,272
Total	337,716	350,147
Residential Mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,749,473	1,667,864
Nonaccrual	46,220	61,240
Total	1,949,832	1,876,893
Residential Mortgage [Member] | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	979,386	913,062
Nonaccrual	22,855	28,984
Total	1,006,820	945,336
Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	40,594	33,653
Nonaccrual	11,846	21,900
Total	199,387	196,937
Residential Mortgage [Member] | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	729,493	721,149
Nonaccrual	11,519	10,356
Total	743,625	734,620
Personal [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	838,811	551,533
Nonaccrual	290	463
Total	839,958	552,697
30 to 59 Days Past Due [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	28,603	36,703
30 to 59 Days Past Due [Member] | Commercial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	4,968	5,269
30 to 59 Days Past Due [Member] | Commercial [Member] | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	479	2,635
30 to 59 Days Past Due [Member] | Commercial [Member] | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	191	66
30 to 59 Days Past Due [Member] | Commercial [Member] | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	3,761	49
30 to 59 Days Past Due [Member] | Commercial [Member] | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	382	0
30 to 59 Days Past Due [Member] | Commercial [Member] | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	2,435
30 to 59 Days Past Due [Member] | Commercial [Member] | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	155	84
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	6	8,114
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	8,114
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	6	0
30 to 59 Days Past Due [Member] | Residential Mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	23,040	22,655
30 to 59 Days Past Due [Member] | Residential Mortgage [Member] | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	3,299	3,290
30 to 59 Days Past Due [Member] | Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	17,465	16,986
30 to 59 Days Past Due [Member] | Residential Mortgage [Member] | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	2,276	2,379
30 to 59 Days Past Due [Member] | Personal [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	589	665
60 to 89 Days Past Due [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	16,748	18,182
60 to 89 Days Past Due [Member] | Commercial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,065	4,041
60 to 89 Days Past Due [Member] | Commercial [Member] | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial [Member] | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,021	4,025
60 to 89 Days Past Due [Member] | Commercial [Member] | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial [Member] | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial [Member] | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	41	0
60 to 89 Days Past Due [Member] | Commercial [Member] | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	3	16
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member] | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member] | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member] | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member] | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member] | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member] | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Residential Mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	15,420	14,113
60 to 89 Days Past Due [Member] | Residential Mortgage [Member] | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,280	0
60 to 89 Days Past Due [Member] | Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	13,803	13,397
60 to 89 Days Past Due [Member] | Residential Mortgage [Member] | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	337	716
60 to 89 Days Past Due [Member] | Personal [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	263	28
90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	115,684	112,208
90 Days or More [Member] | Commercial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	802
90 Days or More [Member] | Commercial [Member] | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
90 Days or More [Member] | Commercial [Member] | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
90 Days or More [Member] | Commercial [Member] | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	700
90 Days or More [Member] | Commercial [Member] | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
90 Days or More [Member] | Commercial [Member] | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
90 Days or More [Member] | Commercial [Member] | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	102
90 Days or More [Member] | Commercial Real Estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	377
90 Days or More [Member] | Commercial Real Estate [Member] | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
90 Days or More [Member] | Commercial Real Estate [Member] | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
90 Days or More [Member] | Commercial Real Estate [Member] | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
90 Days or More [Member] | Commercial Real Estate [Member] | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
90 Days or More [Member] | Commercial Real Estate [Member] | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
90 Days or More [Member] | Commercial Real Estate [Member] | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	377
90 Days or More [Member] | Residential Mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	115,679	111,021
90 Days or More [Member] | Residential Mortgage [Member] | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
90 Days or More [Member] | Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	115,679	111,001
90 Days or More [Member] | Residential Mortgage [Member] | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	20
90 Days or More [Member] | Personal [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	$ 5	$ 8