Loans and Allowance for Credit Losses Credit Quality Indicators (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	$ 12,384,755	$ 12,154,209
Commercial Loans, Commercial Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	8,923,037	8,791,394
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	4,729,930	4,664,426
Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,658,483	3,612,065
Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,466,234	3,413,444
Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	192,249	198,621
Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	534,624	514,903
Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	287,966	288,553
Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	246,658	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	8,063,760	7,891,757
Percentage of total loans rated	90.40%	89.80%
Pass [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	4,311,347	4,255,334
Pass [Member] | Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,351,903	3,268,415
Pass [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,168,623	3,081,215
Pass [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	183,280	187,200
Pass [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	400,510	368,008
Pass [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	167,745	156,538
Pass [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	232,765	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	353,905	355,571
Percentage of total loans rated	3.90%	4.00%
Special Mention [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	181,873	157,640
Special Mention [Member] | Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	119,822	142,698
Special Mention [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	114,558	137,277
Special Mention [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	5,264	5,421
Special Mention [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	52,210	55,233
Special Mention [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	49,784	52,434
Special Mention [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	2,426	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	505,372	544,066
Percentage of total loans rated	5.70%	6.20%
Substandard [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	236,710	251,452
Substandard [Member] | Commercial - industrial, financial, and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	186,758	200,952
Substandard [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	183,053	194,952
Substandard [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	3,705	6,000
Substandard [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	81,904	91,662
Substandard [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	70,437	79,581
Substandard [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans, gross of unearned income	$ 11,467	$ 12,081