Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 645,903	$ 665,646
Fair values	637,883	671,441
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	62,837	71,581
Fair values	62,831	72,669
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	347,379	351,477
Fair values	342,595	357,933
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	222,740	229,750
Fair values	219,932	228,237
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	12,947	12,838
Fair values	$ 12,525	$ 12,602