Investment Securities - Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Amortized Cost
Within one year	$ 83,965
After one year but within five years	181,873
Amortized Cost	265,838	$ 251,810
Fair value
Within one year	84,173
After one year but within five years	184,866
Total fair value	269,039	$ 250,764
Fair value
Within one year	32,977
After one year but within five years	76,645
After five years but within 10 years	29,655
Total fair value	$ 139,277