Investment Securities (Summary of Amortized Cost and Fair Value of Held-To-Maturity Investment Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	$ 518,300	$ 509,971	$ 381,450
Investment securities held-to-maturity, Unrealized Gains	9,493	7,446	6,852
Investment securities held-to-maturity, Unrealized Losses	2,200	4,712	8,126
Total Held-to-maturity securities, Debt maturities, Fair Value	525,593	512,705	380,176
State and political subdivisions [Member]
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	517,800	509,471	370,950
Investment securities held-to-maturity, Unrealized Gains	9,493	7,446	6,852
Investment securities held-to-maturity, Unrealized Losses	1,985	4,512	4,926
Total Held-to-maturity securities, Debt maturities, Fair Value	525,308	512,405	372,876
Trust Preferred Securities [Member]
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	500	500	10,500
Investment securities held-to-maturity, Unrealized Gains	0	0	0
Investment securities held-to-maturity, Unrealized Losses	215	200	3,200
Total Held-to-maturity securities, Debt maturities, Fair Value	$ 285	$ 300	$ 7,300