Investments in debt and equity securities available-for-sale - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	$ 582,967
Equity securities, Amortized costs	29,854	$ 27,497
Total, Amortized costs	612,821	565,384
Debt securities, Fair values	609,266
Equity securities, Fair values	34,425	31,253
Total securities available-for-sale, at fair value	643,691	579,849
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	72,725	70,300
Debt securities, Fair values	75,421	72,008
Corporate
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	341,005	303,870
Debt securities, Fair values	358,164	309,461
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	156,444	149,914
Debt securities, Fair values	162,798	153,221
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	12,793	13,803
Debt securities, Fair values	$ 12,883	$ 13,906