Investments in debt and equity securities - Amortized Costs and Fair Values (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	$ 589,179
Equity securities, Amortized costs	30,255	$ 27,497
Total, Amortized costs	619,434	565,384
Debt securities, Fair values	595,119
Equity securities, Fair values	36,384	31,253
Available for sale securities, Total	631,503	579,849
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	72,284	70,300
Debt securities, Fair values	72,432	72,008
Corporate
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	338,365	303,870
Debt securities, Fair values	343,047	309,461
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	165,735	149,914
Debt securities, Fair values	167,027	153,221
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	12,795	13,803
Debt securities, Fair values	$ 12,613	$ 13,906