Loans and Allowance for Loan Losses (Loan Portfolio Credit Exposure) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 24,456,550	$ 23,882,382	$ 23,088,535
Accrual substandard loans	235,800	256,600
Deferred fees and costs, net	$ 26,000	26,000	$ 27,600
Maximum
Financing Receivable, Recorded Investment [Line Items]
Allowance for loan losses, percent of loan amount	50.00%
Total commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 7,414,271	7,365,974
Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,035,663	5,869,261
1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	835,620	887,307
Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	542,988	609,406
Total commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,750,908	11,551,943
Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,000,573	6,915,927
Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,750,335	4,636,016
Total consumer
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,291,371	4,964,465
Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,563,167	1,617,265
Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,470,665	2,296,604
Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	225,900	232,413
Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,031,639	818,183
Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	23,758,754	23,147,814
Pass | Total commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,218,925	7,135,790
Pass | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,952,286	5,794,626
Pass | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	788,665	826,311
Pass | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	477,974	514,853
Pass | Total commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,300,452	11,105,068
Pass | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,710,038	6,642,648
Pass | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,590,414	4,462,420
Pass | Total consumer
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,239,377	4,906,956
Pass | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,535,583	1,589,199
Pass | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,450,658	2,271,916
Pass | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	224,643	230,997
Pass | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,028,493	814,844
Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	302,709	324,593
Special Mention | Total commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	126,196	137,469
Special Mention | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	61,451	43,336
Special Mention | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	24,169	33,928
Special Mention | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	40,576	60,205
Special Mention | Total commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	176,513	187,124
Special Mention | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	124,412	126,268
Special Mention | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	52,101	60,856
Special Mention | Total consumer
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Special Mention | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Special Mention | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Special Mention | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Special Mention | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	380,069	387,117
Substandard | Total commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	65,712	85,450
Substandard | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	21,926	31,299
Substandard | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	22,559	26,790
Substandard | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	21,227	27,361
Substandard | Total commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	266,764	251,755
Substandard | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	160,354	140,425
Substandard | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	106,410	111,330
Substandard | Total consumer
Financing Receivable, Recorded Investment [Line Items]
Total loans	47,593	49,912
Substandard | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	24,812	22,774
Substandard | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	19,528	23,268
Substandard | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	445	637
Substandard | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,808	3,233
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,462	19,337
Doubtful | Total commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,438	7,265
Doubtful | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Doubtful | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	227	278
Doubtful | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,211	6,987
Doubtful | Total commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,039	7,855
Doubtful | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,629	6,445
Doubtful | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,410	1,410
Doubtful | Total consumer
Financing Receivable, Recorded Investment [Line Items]
Total loans	985	4,217
Doubtful | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	373	2,892
Doubtful | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	313	1,283
Doubtful | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Doubtful | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	299	42
Loss
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,556	3,521
Loss | Total commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Loss | Investment properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Loss | 1-4 family properties
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Loss | Land and development
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Loss | Total commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	140	141
Loss | Commercial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans	140	141
Loss | Owner-occupied
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Loss | Total consumer
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,416	3,380
Loss | Home equity lines
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,399	2,400
Loss | Consumer mortgages
Financing Receivable, Recorded Investment [Line Items]
Total loans	166	137
Loss | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Total loans	812	779
Loss | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 39	$ 64