Loans and Allowances for Credit Losses, By Aging Category (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Financing receivable, recorded investment, aging [Abstract]
Current	$ 17,691,210	$ 16,770,208	$ 16,783,348
Nonaccrual	165,657	187,874	245,439
Total	18,003,696	17,153,424	17,183,645
Commercial
Financing receivable, recorded investment, aging [Abstract]
Current	11,210,170	10,575,384	10,438,108
Nonaccrual	120,978	137,303	197,157
Total	11,349,039	10,733,975	10,637,955
Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	3,081,622	2,833,668	2,723,248
Nonaccrual	65,597	92,284	123,992
Total	3,147,219	2,930,156	2,847,240
Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,937,699	2,983,222	2,949,562
Nonaccrual	4,377	2,620	7,754
Total	2,944,499	2,986,949	2,958,827
Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,685,175	1,468,284	1,532,986
Nonaccrual	14,095	2,574	10,620
Total	1,699,554	1,471,256	1,543,695
Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	644,825	490,739	536,481
Nonaccrual	2,991	5,962	9,656
Total	647,816	496,774	546,137
Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,322,580	2,284,770	2,196,088
Nonaccrual	16,125	14,765	24,505
Total	2,353,722	2,314,753	2,221,518
Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	538,269	514,701	499,743
Nonaccrual	17,793	19,098	20,630
Total	556,229	534,087	520,538
Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Current	3,710,202	3,476,163	3,684,343
Nonaccrual	1,996	2,855	3,775
Total	3,712,220	3,479,987	3,688,592
Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	118,649	115,213	139,070
Nonaccrual	350	1,832	2,051
Total	118,999	117,245	141,592
Commercial real estate | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	766,956	691,256	722,504
Nonaccrual	1,068	276	301
Total	768,024	691,532	722,805
Commercial real estate | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	819,852	831,118	862,577
Nonaccrual	275	275	396
Total	820,127	831,770	862,973
Commercial real estate | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,056,984	979,625	952,370
Nonaccrual	0	0	10
Total	1,056,984	980,017	952,380
Commercial real estate | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	653,384	573,014	693,635
Nonaccrual	0	0	0
Total	653,384	573,014	693,635
Commercial real estate | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	294,377	285,937	314,187
Nonaccrual	303	472	1,017
Total	294,702	286,409	315,207
Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Current	1,771,319	1,754,287	1,744,045
Nonaccrual	42,343	47,447	44,235
Total	1,942,250	1,973,686	1,939,198
Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,041,859	1,014,588	962,443
Nonaccrual	23,105	25,193	23,415
Total	1,068,412	1,043,435	989,040
Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	38,717	22,692	36,867
Nonaccrual	7,567	9,179	9,052
Total	169,653	197,506	191,729
Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	690,743	717,007	744,735
Nonaccrual	11,671	13,075	11,768
Total	704,185	732,745	758,429
Personal
Financing receivable, recorded investment, aging [Abstract]
Current	999,519	964,374	916,852
Nonaccrual	340	269	272
Total	1,000,187	965,776	917,900
30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	21,070	41,991	24,074
30 to 59 Days [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,955	16,215	1,109
30 to 59 Days [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
30 to 59 Days [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,619	514	50
30 to 59 Days [Member] | Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	284	398	89
30 to 59 Days [Member] | Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
30 to 59 Days [Member] | Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	15,218	925
30 to 59 Days [Member] | Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	52	85	45
30 to 59 Days [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	476	474
30 to 59 Days [Member] | Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	200	471
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	254	0
30 to 59 Days [Member] | Commercial real estate | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	22	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	0	0	3
30 to 59 Days [Member] | Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Past Due	18,937	24,619	22,004
30 to 59 Days [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	2,568	3,435	2,024
30 to 59 Days [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	14,757	18,978	18,416
30 to 59 Days [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,612	2,206	1,564
30 to 59 Days [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	178	681	487
60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	29,146	19,529	15,557
60 to 89 Days [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	15,228	4,841	299
60 to 89 Days [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	4,204	0
60 to 89 Days [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	106	486	180
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	0	73	0
60 to 89 Days [Member] | Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	15,017	0	0
60 to 89 Days [Member] | Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	105	78	119
60 to 89 Days [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	370	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	370	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	13,768	14,127	14,969
60 to 89 Days [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	796	219	1,026
60 to 89 Days [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	12,878	13,468	13,581
60 to 89 Days [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	94	440	362
60 to 89 Days [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	150	191	289
90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	96,613	133,822	115,227
90 Days or More [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	708	232	1,282
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	698	107	1,281
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	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	10	125	1
90 Days or More [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	22	123	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	123	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	22	0	0
90 Days or More [Member] | Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Past Due	95,883	133,206	113,945
90 Days or More [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	84	0	132
90 Days or More [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	95,734	133,189	113,813
90 Days or More [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	65	17	0
90 Days or More [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	$ 0	$ 261	$ 0