Loans and Allowances for Credit Losses, By Aging Category (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Financing receivable, recorded investment, aging [Abstract]
Current	$ 16,783,348	$ 16,597,641	$ 16,010,465
Nonaccrual	245,439	230,984	247,184
Total	17,183,645	16,989,660	16,406,749
Commercial
Financing receivable, recorded investment, aging [Abstract]
Current	10,438,108	10,205,838	10,167,077
Nonaccrual	197,157	178,953	181,989
Total	10,637,955	10,390,824	10,356,437
Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,723,248	2,364,890	2,647,678
Nonaccrual	123,992	132,499	168,145
Total	2,847,240	2,497,868	2,818,656
Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,949,562	3,099,605	2,817,217
Nonaccrual	7,754	8,173	9,388
Total	2,958,827	3,108,990	2,830,864
Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	1,532,986	1,561,650	1,530,110
Nonaccrual	10,620	11,407	2,772
Total	1,543,695	1,576,818	1,532,957
Commercial | Manufacturing [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	536,481	509,662	595,110
Nonaccrual	9,656	4,931	293
Total	546,137	514,975	595,403
Commercial | Healthcare [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	2,196,088	2,201,050	2,050,271
Nonaccrual	24,505	825	875
Total	2,221,518	2,201,916	2,051,146
Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	499,743	468,981	526,691
Nonaccrual	20,630	21,118	516
Total	520,538	490,257	527,411
Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Current	3,684,343	3,803,519	3,571,877
Nonaccrual	3,775	5,521	7,780
Total	3,688,592	3,809,046	3,581,966
Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	139,070	132,100	153,315
Nonaccrual	2,051	3,433	4,261
Total	141,592	135,533	157,576
Commercial real estate | Retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	722,504	761,562	794,154
Nonaccrual	301	326	1,265
Total	722,805	761,888	795,419
Commercial real estate | Office [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	862,577	798,462	768,506
Nonaccrual	396	426	606
Total	862,973	798,888	769,112
Commercial real estate | Multifamily [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	952,370	903,234	784,826
Nonaccrual	10	38	65
Total	952,380	903,272	787,200
Commercial real estate | Industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	693,635	871,673	645,510
Nonaccrual	0	76	76
Total	693,635	871,749	645,586
Commercial real estate | Other commercial real estate [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	314,187	336,488	425,566
Nonaccrual	1,017	1,222	1,507
Total	315,207	337,716	427,073
Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Current	1,744,045	1,749,473	1,684,900
Nonaccrual	44,235	46,220	57,061
Total	1,939,198	1,949,832	1,880,923
Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	962,443	979,386	935,857
Nonaccrual	23,415	22,855	27,228
Total	989,040	1,006,820	969,007
Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	36,867	40,594	42,019
Nonaccrual	9,052	11,846	19,741
Total	191,729	199,387	192,732
Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Current	744,735	729,493	707,024
Nonaccrual	11,768	11,519	10,092
Total	758,429	743,625	719,184
Personal
Financing receivable, recorded investment, aging [Abstract]
Current	916,852	838,811	586,611
Nonaccrual	272	290	354
Total	917,900	839,958	587,423
30 to 59 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	24,074	28,603	26,390
30 to 59 Days [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,109	4,968	645
30 to 59 Days [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	479	0
30 to 59 Days [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	50	191	494
30 to 59 Days [Member] | Commercial | Wholesale/retail [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	89	3,761	75
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	925	0	0
30 to 59 Days [Member] | Commercial | Other commercial and industrial [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	45	155	76
30 to 59 Days [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	474	6	2,309
30 to 59 Days [Member] | Commercial real estate | Residential construction and land development [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	471	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	0	0	2,309
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	3	6	0
30 to 59 Days [Member] | Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Past Due	22,004	23,040	23,036
30 to 59 Days [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	2,024	3,299	5,798
30 to 59 Days [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	18,416	17,465	15,349
30 to 59 Days [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,564	2,276	1,889
30 to 59 Days [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	487	589	400
60 to 89 Days [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	15,557	16,748	16,057
60 to 89 Days [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	299	1,065	3,847
60 to 89 Days [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	0
60 to 89 Days [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	180	1,021	3,765
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	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	119	3	82
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	14,969	15,420	12,152
60 to 89 Days [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,026	1,280	124
60 to 89 Days [Member] | Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	13,581	13,803	11,869
60 to 89 Days [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	362	337	159
60 to 89 Days [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	289	263	58
90 Days or More [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	115,227	115,684	106,653
90 Days or More [Member] | Commercial
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,282	0	2,879
90 Days or More [Member] | Commercial | Energy [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	2,833
90 Days or More [Member] | Commercial | Services [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	1,281	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	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	1	0	46
90 Days or More [Member] | Commercial real estate
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	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	0	0	0
90 Days or More [Member] | Residential mortgage
Financing receivable, recorded investment, aging [Abstract]
Past Due	113,945	115,679	103,774
90 Days or More [Member] | Residential mortgage | Permanent mortgage [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	132	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	113,813	115,679	103,754
90 Days or More [Member] | Residential mortgage | Home equity [Member]
Financing receivable, recorded investment, aging [Abstract]
Past Due	0	0	20
90 Days or More [Member] | Personal
Financing receivable, recorded investment, aging [Abstract]
Past Due	$ 0	$ 5	$ 0