Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 599,542	$ 599,412
Fair values	617,278	631,386
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	38,088	45,138
Fair values	39,885	47,603
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	280,090	285,962
Fair values	292,119	305,450
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	274,938	261,748
Fair values	278,808	271,711
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,426	6,564
Fair values	$ 6,466	$ 6,622