Investments in debt and equity securities - Debt Securities According to Contractual Terms (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Amortized costs
In one year or less	$ 53,152
After one year through five years	323,901
After five years through ten years	144,476
After ten years	24,205
Amortized costs, total	545,734	$ 608,686
Fair values
In one year or less	53,176
After one year through five years	330,147
After five years through ten years	152,707
After ten years	25,956
Fair values, total	$ 561,986	$ 602,020