Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 582,054	$ 592,648
Fair values	612,891	605,721
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	47,635	52,176
Fair values	50,068	53,823
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	301,299	299,074
Fair values	319,972	309,142
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	226,561	234,734
Fair values	236,224	236,073
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,559	6,664
Fair values	$ 6,627	$ 6,683