Investment Securities (Summary of the Amortized Cost and Fair Value of Investment Securities by Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment Securities Available-for-Sale: Amortized Cost
Due in one year or less	$ 300,364
Due after one year through five years	751,998
Due after five years through ten years	616,027
Due after ten years	306,868
Preferred stock	1,389
Total	1,976,646
Investment Securities Available-for-Sale: Fair Value
Due in one year or less	298,644
Due after one year through five years	746,082
Due after five years through ten years	611,822
Due after ten years	305,174
Preferred stock	1,824
Total	1,963,546
Investment Securities Held-to-Maturity: Amortized Cost
Due in one year or less	89,359
Due after one year through five years	237,113
Due after five years through ten years	152,299
Due after ten years	198,322
Total	677,093	$ 623,427
Investment Securities Held-to-Maturity: Fair Value
Due in one year or less	89,149
Due after one year through five years	233,022
Due after five years through ten years	148,185
Due after ten years	192,550
Total	$ 662,906	$ 608,531