Loans and Allowance for Credit Losses Credit Quality Indicators (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	$ 12,744,009	$ 12,792,016
Commercial Loans, Commerical Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	9,224,221	9,235,208
Percentage of total loans rated	100.00%	100.00%
Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	5,137,454	5,101,922
Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,574,130	3,628,420
Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,375,041	3,404,163
Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	199,089	224,257
Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	512,637	504,866
Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	229,541	235,369
Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	283,096	269,497
Pass [Member] | Commercial Loans, Commerical Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	8,554,787	8,545,473
Percentage of total loans rated	92.70%	92.60%
Pass [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	4,833,982	4,763,987
Pass [Member] | Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,293,273	3,377,004
Pass [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,109,539	3,167,168
Pass [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	183,734	209,836
Pass [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	427,532	404,482
Pass [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	153,495	146,041
Pass [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	274,037	258,441
Special Mention [Member] | Commercial Loans, Commerical Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	302,945	298,746
Percentage of total loans rated	3.30%	3.20%
Special Mention [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	122,929	141,013
Special Mention [Member] | Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	147,545	123,279
Special Mention [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	137,176	111,613
Special Mention [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	10,369	11,666
Special Mention [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	32,471	34,454
Special Mention [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	29,556	31,522
Special Mention [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	2,915	2,932
Substandard [Member] | Commercial Loans, Commerical Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	366,489	390,989
Percentage of total loans rated	4.00%	4.20%
Substandard [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	180,543	196,922
Substandard [Member] | Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	133,312	128,137
Substandard [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	128,326	125,382
Substandard [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	4,986	2,755
Substandard [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	52,634	65,930
Substandard [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	46,490	57,806
Substandard [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	$ 6,144	$ 8,124