Investments in debt and equity securities available-for-sale - Amortized Costs and Fair Values (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	$ 575,456
Equity securities, Amortized costs	27,634	$ 27,497
Total, Amortized costs	603,090	565,384
Debt securities, Fair values	594,429
Equity securities, Fair values	31,934	31,253
Total, Fair values	626,363	579,849
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	69,999	70,300
Debt securities, Fair values	72,397	72,008
Corporate
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	334,781	303,870
Debt securities, Fair values	346,654	309,461
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	156,875	149,914
Debt securities, Fair values	161,484	153,221
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	13,801	13,803
Debt securities, Fair values	$ 13,894	$ 13,906