Investments in debt and equity securities available-for-sale - Amortized Costs and Fair Values (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	$ 521,979
Equity securities, Amortized costs	29,347	$ 22,710
Total, Amortized costs	551,326	557,380
Debt securities, Fair values	534,519
Equity securities, Fair values	32,264	28,236
Available for sale securities, Total	566,783	580,103
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	61,493	60,656
Debt securities, Fair values	62,022	61,689
Corporate
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	289,187	296,578
Debt securities, Fair values	297,040	308,691
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	157,495	163,099
Debt securities, Fair values	161,346	166,685
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	13,804	14,337
Debt securities, Fair values	$ 14,111	$ 14,802