Investment Securities (Summary of Amortized Cost and Fair Value of Held-To-Maturity Investment Securities) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	$ 282,579	$ 229,977	$ 221,536
Investment securities held-to-maturity, Unrealized Gains	5,978	8,087	8,661
Investment securities held-to-maturity, Unrealized Losses	12,691	8,142	7,012
Total Held-to-maturity securities, Debt maturities, Fair Value	275,866	229,922	223,185
State and political subdivisions [Member]
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	272,079	219,477	211,036
Investment securities held-to-maturity, Unrealized Gains	5,978	8,087	8,661
Investment securities held-to-maturity, Unrealized Losses	8,441	3,367	1,712
Total Held-to-maturity securities, Debt maturities, Fair Value	269,616	224,197	217,985
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	4,250	4,775	5,300
Total Held-to-maturity securities, Debt maturities, Fair Value	$ 6,250	$ 5,725	$ 5,200