Investment Securities (Available for Sale by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized cost
Amortized cost	$ 1,057,119	$ 1,041,868
Fair value
Fair value	1,039,304	$ 1,037,154
Available-for-sale Securities [Member]
Amortized cost
Due in one year or less	7,397
Due after one year through five years	3,008
Due after five years through ten years	24,895
Due after ten years	0
Amortized cost	35,300
Fair value
Due in one year or less	7,392
Due after one year through five years	2,979
Due after five years through ten years	25,330
Due after ten years	0
Fair value	$ 35,701