Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 585,170	$ 608,686
Fair values	590,115	602,020
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	60,467	61,779
Fair values	61,348	61,934
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	322,993	333,289
Fair values	325,918	328,495
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	195,177	200,667
Fair values	196,430	198,938
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,533	12,951
Fair values	$ 6,419	$ 12,653