Investments in debt and equity securities available-for-sale - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Amortized costs
Debt securities	$ 610,284
Equity securities	30,616	$ 30,255
Total, amortized costs	640,900	619,434
Fair values
Debt securities	619,499
Equity securities	36,700	36,384
Total securities available-for-sale, at fair value	656,199	631,503
Municipal
Amortized costs
Debt securities	72,236	72,284
Fair values
Debt securities	72,918	72,432
Corporate
Amortized costs
Debt securities	334,887	338,365
Fair values
Debt securities	341,283	343,047
Foreign
Amortized costs
Debt securities	190,366	165,735
Fair values
Debt securities	192,663	167,027
U.S. Treasury Bonds
Amortized costs
Debt securities	12,795	12,795
Fair values
Debt securities	$ 12,635	$ 12,613