Investment Securities (Maturities of Debt Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Investment Securities Available-for-Sale:
Due in one year or less, Amortized Cost	$ 180,558
Due after one year through five years, Amortized Cost	442,088
Due after five years through ten years, Amortized Cost	50,902
Due after ten years, Amortized Cost	3,003
Preferred stock, Amortized Cost	1,389
Total Available-for-sale Securities, Debt Maturities, Amortized Cost	677,940
Due in one year or less , Fair Value	180,869
Due after one year through five years, Fair Value	443,752
Due after five years through ten years, Fair Value	51,302
Due after ten years, Fair Value	3,029
Preferred stock, Fair Value	1,692
Total Available-for-sale securities, Debt maturities, Fair Value	680,644
Investment Securities Held-to-Maturity:
Due in one year or less, Amortized Cost	35,293
Due after one year through five years, Amortized Cost	171,557
Due after five years through ten years, Amortized Cost	120,799
Due after ten years, Amortized Cost	53,801
Total Held-to-maturity securities, Debt maturities, Amortized Cost	381,450	316,413	278,099
Due in one year or less , Fair Value	35,365
Due after one year through five years, Fair Value	172,699
Due after five years through ten years, Fair Value	122,512
Due after ten years, Fair Value	49,600
Total Held-to-maturity securities, Debt maturities, Fair Value	$ 380,176	$ 315,740	$ 272,407