Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 608,686	$ 665,646
Fair values	602,020	671,441
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	61,779	71,581
Fair values	61,934	72,669
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	333,289	351,477
Fair values	328,495	357,933
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	200,667	229,750
Fair values	198,938	228,237
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	12,951	12,838
Fair values	$ 12,653	$ 12,602