Investment Securities - Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized Cost
Within one year	$ 105,490
After one year but within five years	158,769
Amortized Cost	264,259	$ 251,810
Fair value
Within one year	105,322
After one year but within five years	160,066
Total fair value	265,388	$ 250,764
Fair value
Within one year	29,141
After one year but within five years	51,261
After five years but within 10 years	4,432
Total fair value	$ 84,834