Investment Securities (Summary of Amortized Cost and Fair Value of Held-To-Maturity Investment Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	$ 381,450	$ 316,413	$ 278,099
Investment securities held-to-maturity, Unrealized Gains	6,852	7,294	5,331
Investment securities held-to-maturity, Unrealized Losses	8,126	7,967	11,023
Total Held-to-maturity securities, Debt maturities, Fair Value	380,176	315,740	272,407
State and political subdivisions [Member]
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	370,950	305,913	267,599
Investment securities held-to-maturity, Unrealized Gains	6,852	7,294	5,331
Investment securities held-to-maturity, Unrealized Losses	4,926	4,557	6,983
Total Held-to-maturity securities, Debt maturities, Fair Value	372,876	308,650	265,947
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	3,200	3,410	4,040
Total Held-to-maturity securities, Debt maturities, Fair Value	$ 7,300	$ 7,090	$ 6,460