Investments in debt and equity securities available-for-sale - Amortized Costs and Fair Values (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Equity securities, Amortized costs	$ 23,493	$ 22,710
Total, Amortized costs	546,610	557,380
Debt securities, Fair values	545,360
Equity securities, Fair values	27,216	28,236
Available for sale securities, Total	572,576	580,103
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	61,538	60,656
Debt securities, Fair values	62,807	61,689
Corporate
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	294,666	296,578
Debt securities, Fair values	309,556	308,691
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	153,097	163,099
Debt securities, Fair values	158,769	166,685
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	13,816	14,337
Debt securities, Fair values	$ 14,228	$ 14,802