Loans and Allowances for Credit Losses, By Aging Category (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Financing receivable, recorded investment, aging [Abstract]
Current	$ 16,623,659	$ 16,597,641	$ 15,621,076
Nonaccrual	207,616	230,984	241,830
Total	16,991,906	16,989,660	16,022,566
Commercial
Financing receivable, recorded investment, aging [Abstract]
Current	10,166,025	10,205,838	10,098,325
Nonaccrual	156,825	178,953	174,652
Total	10,327,110	10,390,824	10,288,425
Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,426,100	2,364,890	2,864,408
Nonaccrual	110,425	132,499	159,553
Total	2,537,112	2,497,868	3,029,420
Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,002,984	3,099,605	2,714,697
Nonaccrual	7,713	8,173	9,512
Total	3,013,375	3,108,990	2,728,891
Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,495,023	1,561,650	1,443,503
Nonaccrual	11,090	11,407	3,685
Total	1,506,243	1,576,818	1,451,846
Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	537,168	509,662	600,029
Nonaccrual	5,907	4,931	312
Total	543,430	514,975	600,645
Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,264,266	2,201,050	1,994,402
Nonaccrual	909	825	1,023
Total	2,265,604	2,201,916	1,995,425
Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	440,484	468,981	481,286
Nonaccrual	20,781	21,118	567
Total	461,346	490,257	482,198
Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Current	3,866,234	3,803,519	3,355,981
Nonaccrual	4,475	5,521	9,270
Total	3,871,063	3,809,046	3,370,507
Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	133,064	132,100	167,160
Nonaccrual	2,616	3,433	4,789
Total	135,994	135,533	171,949
Commercial real estate | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	744,732	761,562	809,220
Nonaccrual	314	326	1,302
Total	745,046	761,888	810,522
Commercial real estate | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	860,476	798,462	694,923
Nonaccrual	413	426	629
Total	860,889	798,888	695,552
Commercial real estate | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	922,952	903,234	728,183
Nonaccrual	24	38	250
Total	922,991	903,272	733,689
Commercial real estate | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	871,362	871,673	564,391
Nonaccrual	76	76	76
Total	871,463	871,749	564,467
Commercial real estate | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	333,648	336,488	392,104
Nonaccrual	1,032	1,222	2,224
Total	334,680	337,716	394,328
Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Current	1,744,748	1,749,473	1,673,048
Nonaccrual	46,081	46,220	57,577
Total	1,946,274	1,949,832	1,869,309
Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	948,191	979,386	918,965
Nonaccrual	24,188	22,855	27,497
Total	977,743	1,006,820	948,405
Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	45,643	40,594	43,259
Nonaccrual	10,108	11,846	19,550
Total	204,181	199,387	197,350
Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	750,914	729,493	710,824
Nonaccrual	11,785	11,519	10,530
Total	764,350	743,625	723,554
Personal
Financing receivable, recorded investment, aging [Abstract]
Current	846,652	838,811	493,722
Nonaccrual	235	290	331
Total	847,459	839,958	494,325
30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	40,454	28,603	31,031
30 to 59 Days [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	3,077	4,968	12,607
30 to 59 Days [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	437	479	3,001
30 to 59 Days [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	2,395	191	4,682
30 to 59 Days [Member] | Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	130	3,761	4,658
30 to 59 Days [Member] | Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	115	382	0
30 to 59 Days [Member] | Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
30 to 59 Days [Member] | Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	155	266
30 to 59 Days [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	354	6	0
30 to 59 Days [Member] | Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	314	0	0
30 to 59 Days [Member] | Commercial real estate | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
30 to 59 Days [Member] | Commercial real estate | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
30 to 59 Days [Member] | Commercial real estate | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	15	0	0
30 to 59 Days [Member] | Commercial real estate | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	25	0	0
30 to 59 Days [Member] | Commercial real estate | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	6	0
30 to 59 Days [Member] | Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Past Due	36,593	23,040	18,158
30 to 59 Days [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	5,364	3,299	1,378
30 to 59 Days [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	29,853	17,465	14,835
30 to 59 Days [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,376	2,276	1,945
30 to 59 Days [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	430	589	266
60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,505	16,748	13,394
60 to 89 Days [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,134	1,065	79
60 to 89 Days [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	150	0	0
60 to 89 Days [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	234	1,021	0
60 to 89 Days [Member] | Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
60 to 89 Days [Member] | Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	240	0	0
60 to 89 Days [Member] | Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	429	41	0
60 to 89 Days [Member] | Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	81	3	79
60 to 89 Days [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
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	0
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	266	15,420	13,310
60 to 89 Days [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	1,280	565
60 to 89 Days [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	13,803	12,490
60 to 89 Days [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	266	337	255
60 to 89 Days [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	105	263	5
90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	118,672	115,684	115,235
90 Days or More [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	49	0	2,762
90 Days or More [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	2,458
90 Days or More [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	49	0	0
90 Days or More [Member] | Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
90 Days or More [Member] | Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	304
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	0
90 Days or More [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	5,256
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	5,256
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	0	0	0
90 Days or More [Member] | Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Past Due	118,586	115,679	107,216
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	118,577	115,679	107,216
90 Days or More [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	9	0	0
90 Days or More [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	$ 37	$ 5	$ 1