Investment Securities (Summary of Amortized Cost and Fair Value of Held-To-Maturity Investment Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	$ 257,749	$ 229,977	$ 191,297
Investment securities held-to-maturity, Unrealized Gains	10,002	8,087	7,449
Investment securities held-to-maturity, Unrealized Losses	6,346	8,142	6,374
Investment securities held-to-maturity, Fair Value	261,405	229,922	192,372
State and political subdivisions [Member]
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	247,249	219,477	180,797
Investment securities held-to-maturity, Unrealized Gains	10,002	8,087	7,449
Investment securities held-to-maturity, Unrealized Losses	1,886	3,367	549
Investment securities held-to-maturity, Fair Value	255,365	224,197	187,697
Trust Preferred Securities [Member]
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	10,500	10,500	10,500
Investment securities held-to-maturity, Unrealized Gains	0	0	0
Investment securities held-to-maturity, Unrealized Losses	4,460	4,775	5,825
Investment securities held-to-maturity, Fair Value	$ 6,040	$ 5,725	$ 4,675