Loans and Allowance for Loan Losses (Loan Portfolio Credit Exposure) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 24,906,998	$ 24,812,795	$ 24,284,188
Accrual substandard loans	204,200	190,600
Deferred fees and costs, net	$ 24,000	25,300	$ 25,700
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	$ 24,293,152	24,222,726
Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	289,591	283,865
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	308,500	297,640
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,790	5,358
Loss
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,965	3,206
Total commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,835,727	6,935,288
Total commercial real estate | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,619,050	5,670,065
Total commercial real estate | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	758,904	781,619
Total commercial real estate | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	457,773	483,604
Total commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,682,840	6,763,850
Total commercial real estate | Pass | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,545,689	5,586,792
Total commercial real estate | Pass | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	729,023	745,299
Total commercial real estate | Pass | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	408,128	431,759
Total commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	101,382	117,813
Total commercial real estate | Special Mention | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	53,193	64,628
Total commercial real estate | Special Mention | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	14,331	19,419
Total commercial real estate | Special Mention | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	33,858	33,766
Total commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	48,372	50,496
Total commercial real estate | Substandard | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	20,168	18,645
Total commercial real estate | Substandard | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	15,550	16,901
Total commercial real estate | Substandard | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,654	14,950
Total commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,133	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	3,133	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 commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,101,917	12,023,650
Total commercial and industrial | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,191,531	7,179,487
Total commercial and industrial | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,910,386	4,844,163
Total commercial and industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,674,046	11,643,383
Total commercial and industrial | Pass | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,895,133	6,929,506
Total commercial and industrial | Pass | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,778,913	4,713,877
Total commercial and industrial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	188,209	166,052
Total commercial and industrial | Special Mention | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	132,582	115,912
Total commercial and industrial | Special Mention | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	55,627	50,140
Total commercial and industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	230,636	212,891
Total commercial and industrial | Substandard | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	154,863	132,818
Total commercial and industrial | Substandard | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	75,773	80,073
Total commercial and industrial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	9,026	1,324
Total commercial and industrial | Doubtful | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,953	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 consumer
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,969,354	5,853,857
Total consumer | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,472,471	1,514,227
Total consumer | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,663,371	2,633,503
Total consumer | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	226,713	232,676
Total consumer | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,606,799	1,473,451
Total consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,936,266	5,815,493
Total consumer | Pass | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,452,171	1,491,105
Total consumer | Pass | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,654,285	2,622,499
Total consumer | Pass | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	224,831	230,945
Total consumer | Pass | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,604,979	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	29,492	34,253
Total consumer | Substandard | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	18,421	21,079
Total consumer | Substandard | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,951	10,607
Total consumer | Substandard | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	564	399
Total consumer | Substandard | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,556	2,168
Total consumer | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	631	905
Total consumer | Doubtful | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	271	285
Total consumer | Doubtful | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	103	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	2,965	3,206
Total consumer | Loss | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,608	1,758
Total consumer | Loss | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	32	106
Total consumer | Loss | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,318	1,332
Total consumer | Loss | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 7	$ 10