Loans and Allowances for Credit Losses, By Aging Category (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing receivable, recorded investment, aging [Abstract]
Current	$ 21,308,989	$ 16,770,208
Nonaccrual	163,247	187,874
Total	21,656,730	17,153,424
Commercial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	13,521,894	10,575,384
Nonaccrual	99,841	137,303
Total	13,636,078	10,733,975
Commercial [Member] | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,542,839	2,833,668
Nonaccrual	47,494	92,284
Total	3,590,333	2,930,156
Commercial [Member] | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,231,532	2,518,298
Nonaccrual	8,567	2,620
Total	3,252,146	2,522,025
Commercial [Member] | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,619,290	1,468,284
Nonaccrual	1,316	2,574
Total	1,621,158	1,471,256
Commercial [Member] | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	721,204	490,739
Nonaccrual	8,919	5,962
Total	730,521	496,774
Commercial [Member] | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,716,204	2,213,504
Nonaccrual	16,538	14,765
Total	2,733,537	2,243,487
Commercial [Member] | Public finance [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	876,336	541,775
Nonaccrual	0	0
Total	876,336	541,775
Commercial [Member] | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	814,489	509,116
Nonaccrual	17,007	19,098
Total	832,047	528,502
Commercial Real Estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	4,725,669	3,476,163
Nonaccrual	21,621	2,855
Total	4,764,813	3,479,987
Commercial Real Estate [Member] | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	147,705	115,213
Nonaccrual	350	1,832
Total	148,584	117,245
Commercial Real Estate [Member] | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	884,424	691,256
Nonaccrual	20,279	276
Total	919,082	691,532
Commercial Real Estate [Member] | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,072,920	831,118
Nonaccrual	0	275
Total	1,072,920	831,770
Commercial Real Estate [Member] | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,287,483	979,625
Nonaccrual	301	0
Total	1,288,065	980,017
Commercial Real Estate [Member] | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	776,898	573,014
Nonaccrual	0	0
Total	778,106	573,014
Commercial Real Estate [Member] | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	556,239	285,937
Nonaccrual	691	472
Total	558,056	286,409
Residential Mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,037,128	1,754,287
Nonaccrual	41,555	47,447
Total	2,230,033	1,973,686
Residential Mortgage [Member] | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,292,652	1,014,588
Nonaccrual	23,951	25,193
Total	1,320,165	1,043,435
Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	37,459	22,692
Nonaccrual	7,132	9,179
Total	190,866	197,506
Residential Mortgage [Member] | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	707,017	717,007
Nonaccrual	10,472	13,075
Total	719,002	732,745
Personal [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,024,298	964,374
Nonaccrual	230	269
Total	1,025,806	965,776
30 to 59 Days Past Due [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	53,350	41,991
30 to 59 Days Past Due [Member] | Commercial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	7,675	16,215
30 to 59 Days Past Due [Member] | Commercial [Member] | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
30 to 59 Days Past Due [Member] | Commercial [Member] | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	6,009	514
30 to 59 Days Past Due [Member] | Commercial [Member] | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	515	398
30 to 59 Days Past Due [Member] | Commercial [Member] | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	392	0
30 to 59 Days Past Due [Member] | Commercial [Member] | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	241	15,218
30 to 59 Days Past Due [Member] | Commercial [Member] | Public finance [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
30 to 59 Days Past Due [Member] | Commercial [Member] | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	518	85
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	16,529	476
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	249	200
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	14,379	0
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	254
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	281	22
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,208	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	412	0
30 to 59 Days Past Due [Member] | Residential Mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	28,667	24,619
30 to 59 Days Past Due [Member] | Residential Mortgage [Member] | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	3,196	3,435
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	24,369	18,978
30 to 59 Days Past Due [Member] | Residential Mortgage [Member] | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,102	2,206
30 to 59 Days Past Due [Member] | Personal [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	479	681
60 to 89 Days Past Due [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	24,245	19,529
60 to 89 Days Past Due [Member] | Commercial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	6,106	4,841
60 to 89 Days Past Due [Member] | Commercial [Member] | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	4,204
60 to 89 Days Past Due [Member] | Commercial [Member] | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	6,038	486
60 to 89 Days Past Due [Member] | Commercial [Member] | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	37	0
60 to 89 Days Past Due [Member] | Commercial [Member] | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	6	73
60 to 89 Days Past Due [Member] | Commercial [Member] | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial [Member] | Public finance [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial [Member] | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	25	78
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	280	370
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	280	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	370
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	17,063	14,127
60 to 89 Days Past Due [Member] | Residential Mortgage [Member] | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	366	219
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	16,345	13,468
60 to 89 Days Past Due [Member] | Residential Mortgage [Member] | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	352	440
60 to 89 Days Past Due [Member] | Personal [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	796	191
90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	106,899	133,822
90 Days or More [Member] | Commercial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	562	232
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	107
90 Days or More [Member] | Commercial [Member] | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0
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	554	0
90 Days or More [Member] | Commercial [Member] | Public finance [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	8	125
90 Days or More [Member] | Commercial Real Estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	714	123
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	123
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	714	0
90 Days or More [Member] | Residential Mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	105,620	133,206
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	105,561	133,189
90 Days or More [Member] | Residential Mortgage [Member] | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	59	17
90 Days or More [Member] | Personal [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	$ 3	$ 261