Investments in debt and equity securities available-for-sale - Amortized Costs and Fair Values (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	$ 509,145
Equity securities, Amortized costs	29,805	$ 22,710
Total, Amortized costs	538,950	557,380
Debt securities, Fair values	524,037
Equity securities, Fair values	29,455	28,236
Available for sale securities, Total	553,492	580,103
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	65,789	60,656
Debt securities, Fair values	67,106	61,689
Corporate
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	280,249	296,578
Debt securities, Fair values	289,536	308,691
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	149,303	163,099
Debt securities, Fair values	153,334	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	$ 14,061	$ 14,802