Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 574,448	$ 592,648
Fair values	584,153	605,721
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	48,780	52,176
Fair values	50,348	53,823
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	300,203	299,074
Fair values	302,527	309,142
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	218,918	234,734
Fair values	224,681	236,073
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,547	6,664
Fair values	$ 6,597	$ 6,683