Investment Securities (Summary of Amortized Cost and Fair Value of Held-To-Maturity Investment Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	$ 552,828	$ 509,971	$ 469,837
Investment securities held-to-maturity, Unrealized Gains	16,123	7,446	4,853
Investment securities held-to-maturity, Unrealized Losses	536	4,712	8,112
Total Held-to-maturity securities, Debt maturities, Fair Value	568,415	512,705	466,578
State and political subdivisions [Member]
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	552,328	509,471	469,337
Investment securities held-to-maturity, Unrealized Gains	16,123	7,446	4,853
Investment securities held-to-maturity, Unrealized Losses	321	4,512	7,912
Total Held-to-maturity securities, Debt maturities, Fair Value	568,130	512,405	466,278
Trust Preferred Securities [Member]
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	500	500	500
Investment securities held-to-maturity, Unrealized Gains	0	0	0
Investment securities held-to-maturity, Unrealized Losses	215	200	200
Total Held-to-maturity securities, Debt maturities, Fair Value	$ 285	$ 300	$ 300