Loans and Allowances for Credit Losses, By Aging Category (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Financing receivable, recorded investment, aging [Abstract]
Current	$ 17,006,386	$ 16,770,208	$ 16,623,659
Nonaccrual	180,064	187,874	207,616
Total	17,337,850	17,153,424	16,991,906
Commercial
Financing receivable, recorded investment, aging [Abstract]
Current	10,774,509	10,575,384	10,166,025
Nonaccrual	131,460	137,303	156,825
Total	10,919,667	10,733,975	10,327,110
Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,870,711	2,833,668	2,426,100
Nonaccrual	89,942	92,284	110,425
Total	2,969,618	2,930,156	2,537,112
Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,922,639	2,983,222	3,002,984
Nonaccrual	2,109	2,620	7,713
Total	2,928,294	2,986,949	3,013,375
Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,528,343	1,468,284	1,495,023
Nonaccrual	2,564	2,574	11,090
Total	1,531,576	1,471,256	1,506,243
Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	556,693	490,739	537,168
Nonaccrual	3,002	5,962	5,907
Total	559,695	496,774	543,430
Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,344,134	2,284,770	2,264,266
Nonaccrual	15,342	14,765	909
Total	2,359,928	2,314,753	2,265,604
Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	551,989	514,701	440,484
Nonaccrual	18,501	19,098	20,781
Total	570,556	534,087	461,346
Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Current	3,504,312	3,476,163	3,866,234
Nonaccrual	2,470	2,855	4,475
Total	3,506,782	3,479,987	3,871,063
Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	115,845	115,213	133,064
Nonaccrual	1,613	1,832	2,616
Total	117,458	117,245	135,994
Commercial real estate | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	750,132	691,256	744,732
Nonaccrual	264	276	314
Total	750,396	691,532	745,046
Commercial real estate | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	736,869	831,118	860,476
Nonaccrual	275	275	413
Total	737,144	831,770	860,889
Commercial real estate | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,008,903	979,625	922,952
Nonaccrual	0	0	24
Total	1,008,903	980,017	922,991
Commercial real estate | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	613,608	573,014	871,362
Nonaccrual	0	0	76
Total	613,608	573,014	871,463
Commercial real estate | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	278,955	285,937	333,648
Nonaccrual	318	472	1,032
Total	279,273	286,409	334,680
Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Current	1,763,143	1,754,287	1,744,748
Nonaccrual	45,794	47,447	46,081
Total	1,945,769	1,973,686	1,946,274
Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,020,885	1,014,588	948,191
Nonaccrual	24,578	25,193	24,188
Total	1,047,785	1,043,435	977,743
Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	36,272	22,692	45,643
Nonaccrual	8,883	9,179	10,108
Total	177,880	197,506	204,181
Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	705,986	717,007	750,914
Nonaccrual	12,333	13,075	11,785
Total	720,104	732,745	764,350
Personal
Financing receivable, recorded investment, aging [Abstract]
Current	964,422	964,374	846,652
Nonaccrual	340	269	235
Total	965,632	965,776	847,459
30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	34,769	41,991	40,454
30 to 59 Days [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	4,139	16,215	3,077
30 to 59 Days [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	265	0	437
30 to 59 Days [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	3,487	514	2,395
30 to 59 Days [Member] | Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	234	398	130
30 to 59 Days [Member] | Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	115
30 to 59 Days [Member] | Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	113	15,218	0
30 to 59 Days [Member] | Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	40	85	0
30 to 59 Days [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	476	354
30 to 59 Days [Member] | Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	200	314
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	15
30 to 59 Days [Member] | Commercial real estate | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	25
30 to 59 Days [Member] | Commercial real estate | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
30 to 59 Days [Member] | Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Past Due	29,836	24,619	36,593
30 to 59 Days [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	2,322	3,435	5,364
30 to 59 Days [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	26,137	18,978	29,853
30 to 59 Days [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,377	2,206	1,376
30 to 59 Days [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	794	681	430
60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	9,953	19,529	1,505
60 to 89 Days [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	9,553	4,841	1,134
60 to 89 Days [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	8,700	4,204	150
60 to 89 Days [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	59	486	234
60 to 89 Days [Member] | Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	435	0	0
60 to 89 Days [Member] | Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	73	240
60 to 89 Days [Member] | Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	339	0	429
60 to 89 Days [Member] | Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	20	78	81
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	386	14,127	266
60 to 89 Days [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	219	0
60 to 89 Days [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	13,468	0
60 to 89 Days [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	386	440	266
60 to 89 Days [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	14	191	105
90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	106,678	133,822	118,672
90 Days or More [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	6	232	49
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	107	49
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	6	125	0
90 Days or More [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	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	0	0	0
90 Days or More [Member] | Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Past Due	106,610	133,206	118,586
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	106,588	133,189	118,577
90 Days or More [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	22	17	9
90 Days or More [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	$ 62	$ 261	$ 37