Investment Securities - Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost
Within one year	$ 98,498
After one year but within five years	156,479
Amortized Cost	254,977	$ 251,810
Fair value
Within one year	98,485
After one year but within five years	159,304
Total fair value	257,789	$ 250,764
Fair value
Within one year	31,432
After one year but within five years	73,865
After five years but within 10 years	21,230
Total fair value	$ 126,527