Investments in debt and equity securities - Amortized Costs and Fair Values (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	$ 537,887
Equity securities, Amortized costs	27,497	$ 22,710
Total, Amortized costs	565,384	557,380
Debt securities, fair values	548,596
Equity securities, Fair values	31,253	28,236
Available for sale securities, Total	579,849	580,103
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	70,300	60,656
Debt securities, fair values	72,008	61,689
Corporate
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	303,870	296,578
Debt securities, fair values	309,461	308,691
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	149,914	163,099
Debt securities, fair values	153,221	166,685
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	13,803	14,337
Debt securities, fair values	$ 13,906	$ 14,802