Loans and Allowances for Credit Losses, By Aging Category (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Financing receivable, recorded investment, aging [Abstract]
Current	$ 16,803,397	$ 16,597,641	$ 16,061,958
Nonaccrual	225,636	230,984	236,952
Total	17,206,834	16,989,660	16,464,786
Commercial
Financing receivable, recorded investment, aging [Abstract]
Current	10,606,851	10,205,838	9,926,386
Nonaccrual	176,900	178,953	176,464
Total	10,795,934	10,390,824	10,120,163
Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,752,259	2,364,890	2,365,850
Nonaccrual	110,683	132,499	142,966
Total	2,867,981	2,497,868	2,520,804
Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,963,746	3,099,605	2,923,874
Nonaccrual	1,174	8,173	8,477
Total	2,967,513	3,108,990	2,936,599
Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,654,018	1,561,650	1,599,356
Nonaccrual	1,893	11,407	2,453
Total	1,658,098	1,576,818	1,602,030
Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	508,231	509,662	499,212
Nonaccrual	9,059	4,931	274
Total	519,446	514,975	499,486
Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,214,849	2,201,050	2,083,556
Nonaccrual	24,446	825	855
Total	2,239,451	2,201,916	2,085,046
Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	513,748	468,981	454,538
Nonaccrual	29,645	21,118	21,439
Total	543,445	490,257	476,198
Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Current	3,513,942	3,803,519	3,777,344
Nonaccrual	2,975	5,521	7,350
Total	3,518,142	3,809,046	3,793,598
Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	109,994	132,100	156,207
Nonaccrual	1,924	3,433	3,739
Total	112,102	135,533	159,946
Commercial real estate | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	724,850	761,562	796,362
Nonaccrual	289	326	1,249
Total	725,865	761,888	801,377
Commercial real estate | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	796,687	798,462	751,823
Nonaccrual	275	426	882
Total	797,089	798,888	752,705
Commercial real estate | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	999,009	903,234	868,591
Nonaccrual	0	38	51
Total	999,009	903,272	873,773
Commercial real estate | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	591,080	871,673	837,945
Nonaccrual	0	76	76
Total	591,080	871,749	838,021
Commercial real estate | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	292,322	336,488	366,416
Nonaccrual	487	1,222	1,353
Total	292,997	337,716	367,776
Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Current	1,739,236	1,749,473	1,681,034
Nonaccrual	45,506	46,220	52,452
Total	1,945,750	1,949,832	1,872,793
Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	985,183	979,386	939,853
Nonaccrual	24,623	22,855	25,956
Total	1,013,965	1,006,820	969,558
Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	25,169	40,594	41,150
Nonaccrual	8,891	11,846	15,432
Total	187,370	199,387	190,309
Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	728,884	729,493	700,031
Nonaccrual	11,992	11,519	11,064
Total	744,415	743,625	712,926
Personal
Financing receivable, recorded investment, aging [Abstract]
Current	943,368	838,811	677,194
Nonaccrual	255	290	686
Total	947,008	839,958	678,232
30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	32,750	28,603	39,781
30 to 59 Days [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	4,299	4,968	13,380
30 to 59 Days [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	479	11,988
30 to 59 Days [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	2,343	191	502
30 to 59 Days [Member] | Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,748	3,761	221
30 to 59 Days [Member] | Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	382	0
30 to 59 Days [Member] | Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	156	0	635
30 to 59 Days [Member] | Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	52	155	34
30 to 59 Days [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,038	6	3,773
30 to 59 Days [Member] | Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	184	0	0
30 to 59 Days [Member] | Commercial real estate | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	726	0	3,766
30 to 59 Days [Member] | Commercial real estate | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	127	0	0
30 to 59 Days [Member] | Commercial real estate | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
30 to 59 Days [Member] | Commercial real estate | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
30 to 59 Days [Member] | Commercial real estate | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1	6	7
30 to 59 Days [Member] | Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Past Due	24,117	23,040	22,437
30 to 59 Days [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	3,705	3,299	3,547
30 to 59 Days [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	17,346	17,465	17,364
30 to 59 Days [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	3,066	2,276	1,526
30 to 59 Days [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	3,296	589	191
60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	22,177	16,748	18,856
60 to 89 Days [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	7,854	1,065	107
60 to 89 Days [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	5,039	0	0
60 to 89 Days [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	250	1,021	39
60 to 89 Days [Member] | Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	409	0	0
60 to 89 Days [Member] | Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	2,156	0	0
60 to 89 Days [Member] | Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	41	0
60 to 89 Days [Member] | Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	3	68
60 to 89 Days [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	5,131
60 to 89 Days [Member] | Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
60 to 89 Days [Member] | Commercial real estate | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
60 to 89 Days [Member] | Commercial real estate | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
60 to 89 Days [Member] | Commercial real estate | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	5,131
60 to 89 Days [Member] | Commercial real estate | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
60 to 89 Days [Member] | Commercial real estate | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
60 to 89 Days [Member] | Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Past Due	14,242	15,420	13,470
60 to 89 Days [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	454	1,280	202
60 to 89 Days [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	13,343	13,803	12,963
60 to 89 Days [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	445	337	305
60 to 89 Days [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	81	263	148
90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	122,874	115,684	107,239
90 Days or More [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	30	0	3,826
90 Days or More [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
90 Days or More [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	3,707
90 Days or More [Member] | Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	30	0	0
90 Days or More [Member] | Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
90 Days or More [Member] | Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
90 Days or More [Member] | Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	119
90 Days or More [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	187	0	0
90 Days or More [Member] | Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
90 Days or More [Member] | Commercial real estate | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
90 Days or More [Member] | Commercial real estate | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
90 Days or More [Member] | Commercial real estate | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
90 Days or More [Member] | Commercial real estate | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
90 Days or More [Member] | Commercial real estate | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	187	0	0
90 Days or More [Member] | Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Past Due	122,649	115,679	103,400
90 Days or More [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
90 Days or More [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	122,621	115,679	103,400
90 Days or More [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	28	0	0
90 Days or More [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	$ 8	$ 5	$ 13