Loans and Allowance for Credit Losses Credit Quality Indicators (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	$ 12,789,675	$ 12,154,209
Commercial Loans, Commercial Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	9,212,541	8,791,393
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	5,063,373	4,664,426
Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,645,270	3,612,065
Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,439,534	3,413,444
Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	205,736	198,621
Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	503,898	514,902
Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	245,057	288,552
Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	258,841	226,350
Pass [Member] | Commercial Loans, Commercial Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	8,456,834	7,891,756
Percentage of total loans rated	91.80%	89.80%
Pass [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	4,682,593	4,255,334
Pass [Member] | Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,381,068	3,268,415
Pass [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,183,136	3,081,215
Pass [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	197,932	187,200
Pass [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	393,173	368,007
Pass [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	145,823	156,537
Pass [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	247,350	211,470
Special Mention [Member] | Commercial Loans, Commercial Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	321,030	355,571
Percentage of total loans rated	3.50%	4.00%
Special Mention [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	169,811	157,640
Special Mention [Member] | Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	107,073	142,698
Special Mention [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	103,085	137,277
Special Mention [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,988	5,421
Special Mention [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	44,146	55,233
Special Mention [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	41,776	52,434
Special Mention [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	2,370	2,799
Substandard [Member] | Commercial Loans, Commercial Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	434,677	544,066
Percentage of total loans rated	4.70%	6.20%
Substandard [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	210,969	251,452
Substandard [Member] | Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	157,129	200,952
Substandard [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	153,313	194,952
Substandard [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,816	6,000
Substandard [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	66,579	91,662
Substandard [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	57,458	79,581
Substandard [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	$ 9,121	$ 12,081