Fair value measurements (Details) $ in Thousands | Mar. 31, 2017USD ($)input | Dec. 31, 2016USD ($) |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | $ 656,199 | $ 631,503 |
Corporate | Level 2 | Minimum | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Number of inputs used in fair value measurement | input | 3 | |
Corporate | Level 2 | Maximum | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Number of inputs used in fair value measurement | input | 10 | |
Fair Value, Measurements, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | $ 656,199 | 631,503 |
Fair Value, Measurements, Recurring | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | 36,700 | 36,384 |
Fair Value, Measurements, Recurring | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | 619,499 | 595,119 |
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 | 72,918 | 72,432 |
Fair Value, Measurements, Recurring | Municipal | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | 0 | 0 |
Fair Value, Measurements, Recurring | Municipal | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | 72,918 | 72,432 |
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 | 341,283 | 343,047 |
Fair Value, Measurements, Recurring | Corporate | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | 0 | 0 |
Fair Value, Measurements, Recurring | Corporate | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | 341,283 | 343,047 |
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 | 192,663 | 167,027 |
Fair Value, Measurements, Recurring | Foreign | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | 0 | 0 |
Fair Value, Measurements, Recurring | Foreign | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | 192,663 | 167,027 |
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 | 12,635 | 12,613 |
Fair Value, Measurements, Recurring | U.S. Treasury Bonds | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | 0 | 0 |
Fair Value, Measurements, Recurring | U.S. Treasury Bonds | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | 12,635 | 12,613 |
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 | 36,700 | 36,384 |
Fair Value, Measurements, Recurring | Equity securities | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | 36,700 | 36,384 |
Fair Value, Measurements, Recurring | Equity securities | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | $ 0 | $ 0 |