Investments - Marketable securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost, Current	$ 10,820	$ 1,525
Municipal Bonds [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost, Current	10,820	1,525
Amortized Cost, Due from one through five years	9,678	9,752
Amortized Cost, Due from six through ten years	5,032	6,239
Amortized Cost, Due from eleven through twenty years	3,256	2,711
Amortized Cost, Total	28,786	20,227
Market Value, Current	10,838	1,532
Market Value, Due from one through five years	9,925	9,861
Market Value, Due from six through ten years	5,397	6,433
Market Value, Due from eleven through twenty years	3,422	2,713
Market Value, Total	$ 29,582	$ 20,539