Loans and Allowance for Loan Losses (Loan Portfolio Credit Exposure) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 25,156,773	$ 24,812,795	$ 24,456,550
Accrual substandard loans	209,600	190,600
Deferred fees and costs, net	$ 22,700	25,300	$ 26,000
Maximum
Financing Receivable, Recorded Investment [Line Items]
Allowance for loan losses, percent of loan amount	50.00%
Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 24,569,082	24,222,726
Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	260,760	283,865
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	317,888	297,640
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,903	5,358
Loss
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,140	3,206
Total commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,275,472	12,023,650
Total commercial and industrial | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,271,080	7,179,487
Total commercial and industrial | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,004,392	4,844,163
Total commercial and industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,870,017	11,643,383
Total commercial and industrial | Pass | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,996,081	6,929,506
Total commercial and industrial | Pass | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,873,936	4,713,877
Total commercial and industrial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	170,624	166,052
Total commercial and industrial | Special Mention | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	107,251	115,912
Total commercial and industrial | Special Mention | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	63,373	50,140
Total commercial and industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	231,591	212,891
Total commercial and industrial | Substandard | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	164,581	132,818
Total commercial and industrial | Substandard | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	67,010	80,073
Total commercial and industrial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,240	1,324
Total commercial and industrial | Doubtful | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,167	1,251
Total commercial and industrial | Doubtful | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	73	73
Total commercial and industrial | Loss
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total commercial and industrial | Loss | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total commercial and industrial | Loss | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,644,171	6,935,288
Total commercial real estate | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,509,596	5,670,065
Total commercial real estate | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	720,710	781,619
Total commercial real estate | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	413,865	483,604
Total commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,490,330	6,763,850
Total commercial real estate | Pass | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,422,727	5,586,792
Total commercial real estate | Pass | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	698,532	745,299
Total commercial real estate | Pass | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	369,071	431,759
Total commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	90,136	117,813
Total commercial real estate | Special Mention | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	51,279	64,628
Total commercial real estate | Special Mention | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	9,245	19,419
Total commercial real estate | Special Mention | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	29,612	33,766
Total commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	60,576	50,496
Total commercial real estate | Substandard | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	35,590	18,645
Total commercial real estate | Substandard | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,933	16,901
Total commercial real estate | Substandard | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,053	14,950
Total commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,129	3,129
Total commercial real estate | Doubtful | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total commercial real estate | Doubtful | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total commercial real estate | Doubtful | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,129	3,129
Total commercial real estate | Loss
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total commercial real estate | Loss | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total commercial real estate | Loss | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total commercial real estate | Loss | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total consumer
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,237,130	5,853,857
Total consumer | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,453,855	1,514,227
Total consumer | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,750,935	2,633,503
Total consumer | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	238,424	232,676
Total consumer | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,793,916	1,473,451
Total consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,208,735	5,815,493
Total consumer | Pass | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,435,724	1,491,105
Total consumer | Pass | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,743,245	2,622,499
Total consumer | Pass | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	237,198	230,945
Total consumer | Pass | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,792,568	1,470,944
Total consumer | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total consumer | Special Mention | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total consumer | Special Mention | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total consumer | Special Mention | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total consumer | Special Mention | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total consumer | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	25,721	34,253
Total consumer | Substandard | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	16,599	21,079
Total consumer | Substandard | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,588	10,607
Total consumer | Substandard | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	447	399
Total consumer | Substandard | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,087	2,168
Total consumer | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	534	905
Total consumer | Doubtful | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	175	285
Total consumer | Doubtful | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	102	291
Total consumer | Doubtful | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Total consumer | Doubtful | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	257	329
Total consumer | Loss
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,140	3,206
Total consumer | Loss | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,357	1,758
Total consumer | Loss | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	106
Total consumer | Loss | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	779	1,332
Total consumer | Loss | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 4	$ 10