Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 599,412	$ 592,648
Fair values	631,386	605,721
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	45,138	52,176
Fair values	47,603	53,823
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	285,962	299,074
Fair values	305,450	309,142
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	261,748	234,734
Fair values	271,711	236,073
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,564	6,664
Fair values	$ 6,622	$ 6,683