Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	$ 665,646
Equity securities, Amortized costs	32,132	$ 30,255
Amortized costs, Total	697,778	619,434
Debt securities, Fair values	671,441
Equity securities, Fair values	37,914	36,384
Available for sale securities, Total	709,355	631,503
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	71,581	72,284
Debt securities, Fair values	72,669	72,432
Corporate
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	351,477	338,365
Debt securities, Fair values	357,933	343,047
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	229,750	165,735
Debt securities, Fair values	228,237	167,027
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	12,838	12,795
Debt securities, Fair values	$ 12,602	$ 12,613