Loans and Allowance for Credit Losses Present Loans, Net of Unearned Income and Their Related Allowance for Loan Losses, by Portfolio Segment (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses measured for impairment under FASB ASC Subtopic 450-20	$ 113,250		$ 123,544
Allowance for loan losses evaluated for impairment under FASB ASC Section 310-10-35	50,591		54,157
Allowance for loan losses	163,841	$ 169,054	177,701	$ 184,144
Loans, net of unearned income, measured for impairment under FASB ASC Subtopic 450-20	13,688,235		12,912,650
Loans, net of unearned income, evaluated for impairment under FASB ASC Subtopic 310-10-35	182,466		202,855
Loans, net of unearned income	$ 13,870,701	13,838,602	$ 13,115,505
Percentage of unallocated allowance	4.00%		5.00%
Real-estate commercial mortage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses measured for impairment under FASB ASC Subtopic 450-20	$ 35,914		$ 38,916
Allowance for loan losses evaluated for impairment under FASB ASC Section 310-10-35	12,397		13,944
Allowance for loan losses	48,311	47,866	52,860	53,493
Loans, net of unearned income, measured for impairment under FASB ASC Subtopic 450-20	5,499,820		5,157,342
Loans, net of unearned income, evaluated for impairment under FASB ASC Subtopic 310-10-35	58,288		69,759
Loans, net of unearned income	5,558,108	5,462,330	5,227,101
Commercial - industrial, financial, and agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses measured for impairment under FASB ASC Subtopic 450-20	40,969		40,027
Allowance for loan losses evaluated for impairment under FASB ASC Section 310-10-35	13,364		17,123
Allowance for loan losses	54,333	57,098	57,150	51,378
Loans, net of unearned income, measured for impairment under FASB ASC Subtopic 450-20	3,992,567		3,716,037
Loans, net of unearned income, evaluated for impairment under FASB ASC Subtopic 310-10-35	42,766		46,594
Loans, net of unearned income	4,035,333	4,088,962	3,762,631
Home Equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses measured for impairment under FASB ASC Subtopic 450-20	13,541		16,937
Allowance for loan losses evaluated for impairment under FASB ASC Section 310-10-35	8,983		6,544
Allowance for loan losses	22,524	22,405	23,481	28,271
Loans, net of unearned income, measured for impairment under FASB ASC Subtopic 450-20	1,641,457		1,688,869
Loans, net of unearned income, evaluated for impairment under FASB ASC Subtopic 310-10-35	18,024		12,754
Loans, net of unearned income	1,659,481	1,684,439	1,701,623
Real-estate - residential mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses measured for impairment under FASB ASC Subtopic 450-20	7,599		9,162
Allowance for loan losses evaluated for impairment under FASB ASC Section 310-10-35	12,329		14,073
Allowance for loan losses	19,928	21,375	23,235	29,072
Loans, net of unearned income, measured for impairment under FASB ASC Subtopic 450-20	1,329,114		1,312,861
Loans, net of unearned income, evaluated for impairment under FASB ASC Subtopic 310-10-35	48,345		51,927
Loans, net of unearned income	1,377,459	1,376,160	1,364,788
Real-estate - construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses measured for impairment under FASB ASC Subtopic 450-20	4,004		6,037
Allowance for loan losses evaluated for impairment under FASB ASC Section 310-10-35	2,278		2,450
Allowance for loan losses	6,282	6,529	8,487	9,756
Loans, net of unearned income, measured for impairment under FASB ASC Subtopic 450-20	797,282		656,021
Loans, net of unearned income, evaluated for impairment under FASB ASC Subtopic 310-10-35	13,590		21,785
Loans, net of unearned income	810,872	799,988	677,806
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses measured for impairment under FASB ASC Subtopic 450-20	2,302		2,504
Allowance for loan losses evaluated for impairment under FASB ASC Section 310-10-35	22		23
Allowance for loan losses	2,324	2,585	2,527	3,015
Loans, net of unearned income, measured for impairment under FASB ASC Subtopic 450-20	263,189		257,265
Loans, net of unearned income, evaluated for impairment under FASB ASC Subtopic 310-10-35	32		36
Loans, net of unearned income	263,221	268,588	257,301
Leasing and other and overdrafts [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses measured for impairment under FASB ASC Subtopic 450-20	1,756		1,653
Allowance for loan losses evaluated for impairment under FASB ASC Section 310-10-35	1,218		0
Allowance for loan losses	2,974	2,468	1,653	1,799
Loans, net of unearned income, measured for impairment under FASB ASC Subtopic 450-20	164,806		124,255
Loans, net of unearned income, evaluated for impairment under FASB ASC Subtopic 310-10-35	1,421		0
Loans, net of unearned income	166,227	158,135	124,255
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses measured for impairment under FASB ASC Subtopic 450-20	7,165		8,308
Allowance for loan losses	$ 7,165	$ 8,728	$ 8,308	$ 7,360