Fair value measurements - Financial Instrument Measured at Fair Value on Recurring Basis (Detail) (USD $) | Sep. 30, 2014 | Dec. 31, 2013 |
In Thousands, unless otherwise specified |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale securities, Fair value measurements | $565,613 | $537,343 |
Fair Value, Measurements, Recurring | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale securities, Fair value measurements | 565,613 | 537,343 |
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 | 55,186 | 47,252 |
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 | 303,115 | 291,832 |
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 | 170,434 | 162,367 |
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 | 14,369 | 15,197 |
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 | 22,509 | 20,695 |
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 | 22,509 | 20,695 |
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 | 22,509 | 20,695 |
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 | 543,104 | 516,648 |
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 | 55,186 | 47,252 |
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 | 303,115 | 291,832 |
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 | 170,434 | 162,367 |
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 | $14,369 | $15,197 |