Investments in debt and equity securities available-for-sale - Amortized Costs and Fair Values (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Equity securities, Amortized costs	$ 17,663	$ 17,441
Total, Amortized costs	547,068	528,833
Debt securities, Fair values	543,104
Equity securities, Fair values	22,509	20,695
Available for sale securities, Total	565,613	537,343
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	54,209	47,808
Debt securities, Fair values	55,186	47,252
Corporate
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	292,208	285,104
Debt securities, Fair values	303,115	291,832
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	169,169	164,146
Debt securities, Fair values	170,434	162,367
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	13,819	14,334
Debt securities, Fair values	$ 14,369	$ 15,197