Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 592,648	$ 608,686
Fair values	605,721	602,020
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	52,176	61,779
Fair values	53,823	61,934
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	299,074	333,289
Fair values	309,142	328,495
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	234,734	200,667
Fair values	236,073	198,938
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,664	12,951
Fair values	$ 6,683	$ 12,653