Loans and Allowances for Credit Losses, By Aging Category (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Financing receivable, recorded investment, aging [Abstract]
Current	$ 15,626,933	$ 13,933,715
Nonaccrual	147,128	80,771
Total	15,941,154	14,208,037
30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	54,885	41,437
90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	112,208	152,114
Commercial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	10,165,995	9,078,724
Nonaccrual	76,424	13,527
Total	10,252,531	9,095,670
Commercial [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	9,310	3,419
Commercial [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	802	0
Commercial [Member] | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,033,504	2,857,082
Nonaccrual	61,189	1,416
Total	3,097,328	2,860,428
Commercial [Member] | Energy [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	2,635	1,930
Commercial [Member] | Energy [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial [Member] | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,769,895	2,385,193
Nonaccrual	10,290	5,201
Total	2,784,276	2,391,530
Commercial [Member] | Services [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	4,091	1,136
Commercial [Member] | Services [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial [Member] | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,879,873	1,453,409
Nonaccrual	1,072	1,380
Total	1,883,380	1,454,969
Commercial [Member] | Healthcare [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	2,435	180
Commercial [Member] | Healthcare [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial [Member] | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,418,396	1,435,866
Nonaccrual	2,919	4,149
Total	1,422,064	1,440,015
Commercial [Member] | Wholesale/retail [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	49	0
Commercial [Member] | Wholesale/retail [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	700	0
Commercial [Member] | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	556,398	532,144
Nonaccrual	331	450
Total	556,729	532,594
Commercial [Member] | Manufacturing [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial [Member] | Manufacturing [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial [Member] | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	507,929	415,030
Nonaccrual	623	931
Total	508,754	416,134
Commercial [Member] | Other commercial and industrial [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	100	173
Commercial [Member] | Other commercial and industrial [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	102	0
Commercial Real Estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,241,541	2,704,373
Nonaccrual	9,001	18,557
Total	3,259,033	2,728,150
Commercial Real Estate [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	8,114	5,220
Commercial Real Estate [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	377	0
Commercial Real Estate [Member] | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	795,180	662,963
Nonaccrual	1,319	3,926
Total	796,499	666,889
Commercial Real Estate [Member] | Retail [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial Real Estate [Member] | Retail [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial Real Estate [Member] | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	742,697	704,298
Nonaccrual	274	0
Total	751,085	704,298
Commercial Real Estate [Member] | Multifamily [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	8,114	0
Commercial Real Estate [Member] | Multifamily [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial Real Estate [Member] | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	637,056	412,124
Nonaccrual	651	3,420
Total	637,707	415,544
Commercial Real Estate [Member] | Office [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial Real Estate [Member] | Office [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial Real Estate [Member] | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	563,093	428,817
Nonaccrual	76	0
Total	563,169	428,817
Commercial Real Estate [Member] | Industrial [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial Real Estate [Member] | Industrial [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial Real Estate [Member] | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	156,017	133,642
Nonaccrual	4,409	5,299
Total	160,426	143,591
Commercial Real Estate [Member] | Residential construction and land development [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	4,650
Commercial Real Estate [Member] | Residential construction and land development [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
Commercial Real Estate [Member] | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	347,498	362,529
Nonaccrual	2,272	5,912
Total	350,147	369,011
Commercial Real Estate [Member] | Other commercial real estate [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	570
Commercial Real Estate [Member] | Other commercial real estate [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	377	0
Residential Mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,667,864	1,717,028
Nonaccrual	61,240	48,121
Total	1,876,893	1,949,512
Residential Mortgage [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	36,768	32,251
Residential Mortgage [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	111,021	152,112
Residential Mortgage [Member] | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	913,062	929,090
Nonaccrual	28,984	34,845
Total	945,336	969,951
Residential Mortgage [Member] | Permanent mortgage [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	3,290	5,970
Residential Mortgage [Member] | Permanent mortgage [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	46
Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	33,653	26,691
Nonaccrual	21,900	3,712
Total	196,937	205,950
Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	30,383	23,558
Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	111,001	151,989
Residential Mortgage [Member] | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	721,149	761,247
Nonaccrual	10,356	9,564
Total	734,620	773,611
Residential Mortgage [Member] | Home equity [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	3,095	2,723
Residential Mortgage [Member] | Home equity [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	20	77
Personal [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	551,533	433,590
Nonaccrual	463	566
Total	552,697	434,705
Personal [Member] | 30 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	693	547
Personal [Member] | 90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	$ 8	$ 2