Loans and Allowance for Loan Losses (Loan Portfolio Credit Exposure) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 24,284,188	$ 23,882,382	$ 22,787,534
Accrual substandard loans	241,800	256,600
Deferred fees and costs, net	$ 25,700	26,000	$ 29,300
Maximum [Member]
Financing Receivable, Recorded Investment [Line Items]
Allowance for loan losses, percent of loan amount	50.00%
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 7,459,223	7,365,974
Investment Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,016,052	5,869,261
1-4 Family Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	861,271	887,307
Land and Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	581,900	609,406
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,740,766	11,551,943
Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,056,032	6,915,927
Owner-Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,684,734	4,636,016
Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,084,199	4,964,465
Home Equity Lines [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,587,102	1,617,265
Consumer Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,350,730	2,296,604
Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	224,349	232,413
Other Consumer Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	922,018	818,183
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	23,579,944	23,147,814
Pass [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,264,108	7,135,790
Pass [Member] | Investment Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,945,008	5,794,626
Pass [Member] | 1-4 Family Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	812,321	826,311
Pass [Member] | Land and Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	506,779	514,853
Pass [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,287,203	11,105,068
Pass [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,765,031	6,642,648
Pass [Member] | Owner-Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,522,172	4,462,420
Pass [Member] | Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,028,633	4,906,956
Pass [Member] | Home Equity Lines [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,557,077	1,589,199
Pass [Member] | Consumer Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,329,408	2,271,916
Pass [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	222,733	230,997
Pass [Member] | Other Consumer Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	919,415	814,844
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	304,066	324,593
Special Mention [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	115,190	137,469
Special Mention [Member] | Investment Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	46,057	43,336
Special Mention [Member] | 1-4 Family Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	24,801	33,928
Special Mention [Member] | Land and Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	44,332	60,205
Special Mention [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	188,876	187,124
Special Mention [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	138,102	126,268
Special Mention [Member] | Owner-Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	50,774	60,856
Special Mention [Member] | Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Special Mention [Member] | Home Equity Lines [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Special Mention [Member] | Consumer Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Special Mention [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Special Mention [Member] | Other Consumer Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	377,629	387,117
Substandard [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	72,764	85,450
Substandard [Member] | Investment Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	24,987	31,299
Substandard [Member] | 1-4 Family Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	23,901	26,790
Substandard [Member] | Land and Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	23,876	27,361
Substandard [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	256,707	251,755
Substandard [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	146,329	140,425
Substandard [Member] | Owner-Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	110,378	111,330
Substandard [Member] | Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	48,158	49,912
Substandard [Member] | Home Equity Lines [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	25,157	22,774
Substandard [Member] | Consumer Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	20,094	23,268
Substandard [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	391	637
Substandard [Member] | Other Consumer Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,516	3,233
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	18,367	19,337
Doubtful [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,161	7,265
Doubtful [Member] | Investment Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Doubtful [Member] | 1-4 Family Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	248	278
Doubtful [Member] | Land and Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,913	6,987
Doubtful [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,840	7,855
Doubtful [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,430	6,445
Doubtful [Member] | Owner-Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,410	1,410
Doubtful [Member] | Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,366	4,217
Doubtful [Member] | Home Equity Lines [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,263	2,892
Doubtful [Member] | Consumer Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,061	1,283
Doubtful [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Doubtful [Member] | Other Consumer Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	42	42
Loss [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,182	3,521
Loss [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Loss [Member] | Investment Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Loss [Member] | 1-4 Family Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Loss [Member] | Land and Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Loss [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	140	141
Loss [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	140	141
Loss [Member] | Owner-Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Loss [Member] | Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,042	3,380
Loss [Member] | Home Equity Lines [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,605	2,400
Loss [Member] | Consumer Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	167	137
Loss [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,225	779
Loss [Member] | Other Consumer Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 45	$ 64