Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 568,015	$ 608,686
Fair values	579,823	602,020
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	60,141	61,779
Fair values	61,578	61,934
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	311,687	333,289
Fair values	319,745	328,495
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	189,650	200,667
Fair values	192,068	198,938
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,537	12,951
Fair values	$ 6,432	$ 12,653