Investment Securities (Securities Held to Maturity by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized cost
Amortized cost	$ 466,945	$ 477,652
Fair value
Fair value	465,787	$ 485,039
Held-to-maturity Securities [Member]
Amortized cost
Due in one year or less	7,452
Due after one year through five years	71,292
Due after five years through ten years	255,234
Due after ten years	132,656
Amortized cost	466,634
Fair value
Due in one year or less	7,488
Due after one year through five years	71,458
Due after five years through ten years	254,865
Due after ten years	131,654
Fair value	$ 465,465