Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 586,115	$ 592,648
Fair values	617,895	605,721
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	46,397	52,176
Fair values	48,952	53,823
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	301,297	299,074
Fair values	320,348	309,142
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	231,859	234,734
Fair values	241,969	236,073
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,562	6,664
Fair values	$ 6,626	$ 6,683