Investments in debt and equity securities available-for-sale - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized costs
Debt securities	$ 634,493
Equity securities	29,900	$ 30,255
Total investments in debt and equity securities available-for-sale, at amortized cost	664,393	619,434
Fair values
Debt securities	643,403
Equity securities	36,279	36,384
Total investments in debt and equity securities available-for-sale, at fair value	679,682	631,503
Municipal
Amortized costs
Debt securities	71,849	72,284
Fair values
Debt securities	73,355	72,432
Corporate
Amortized costs
Debt securities	334,143	338,365
Fair values
Debt securities	342,760	343,047
Foreign
Amortized costs
Debt securities	215,664	165,735
Fair values
Debt securities	214,567	167,027
U.S. Treasury Bonds
Amortized costs
Debt securities	12,837	12,795
Fair values
Debt securities	$ 12,721	$ 12,613