Fair value measurements - Financial Instrument Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	$ 572,576	$ 580,103
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	572,576	580,103
Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	62,807	61,689
Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	309,556	308,691
Fair Value, Measurements, Recurring | Foreign
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	158,769	166,685
Fair Value, Measurements, Recurring | U.S. Treasury Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	14,228	14,802
Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	27,216	28,236
Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	27,216	28,236
Level 1 | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	27,216	28,236
Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	545,360	551,867
Level 2 | Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	62,807	61,689
Level 2 | Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	309,556	308,691
Level 2 | Fair Value, Measurements, Recurring | Foreign
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	158,769	166,685
Level 2 | Fair Value, Measurements, Recurring | U.S. Treasury Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Fair value measurements	$ 14,228	$ 14,802