Investment Securities (Summary of Amortized Cost and Fair Value of Held-To-Maturity Investment Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	$ 501,083	$ 316,413	$ 318,562
Investment securities held-to-maturity, Unrealized Gains	6,038	7,294	5,704
Investment securities held-to-maturity, Unrealized Losses	7,217	7,967	9,991
Total Held-to-maturity securities, Debt maturities, Fair Value	499,904	315,740	314,275
State and political subdivisions [Member]
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	500,583	305,913	308,062
Investment securities held-to-maturity, Unrealized Gains	6,038	7,294	5,704
Investment securities held-to-maturity, Unrealized Losses	7,017	4,557	6,476
Total Held-to-maturity securities, Debt maturities, Fair Value	499,604	308,650	307,290
Trust Preferred Securities [Member]
Held-to-maturity Securities
Investment securities held-to-maturity, Amortized Cost	500	10,500	10,500
Investment securities held-to-maturity, Unrealized Gains	0	0	0
Investment securities held-to-maturity, Unrealized Losses	200	3,410	3,515
Total Held-to-maturity securities, Debt maturities, Fair Value	$ 300	$ 7,090	$ 6,985