Investments in debt and equity securities - Debt Securities According to Contractual Terms (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized costs
In one year or less	$ 50,945
After one year through five years	326,286
After five years through ten years	176,344
After ten years	31,595
Amortized costs, total	585,170	$ 608,686
Fair values
In one year or less	50,822
After one year through five years	328,615
After five years through ten years	178,348
After ten years	32,330
Fair values, total	$ 590,115	$ 602,020