Fair value measurements (Details) $ in Thousands | Sep. 30, 2016USD ($)input | Dec. 31, 2015USD ($) |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities, fair value measurements | $ 643,691 | $ 579,849 |
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 | $ 643,691 | 579,849 |
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 | 34,425 | 31,253 |
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 | 609,266 | 548,596 |
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 | 75,421 | 72,008 |
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 | 75,421 | 72,008 |
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 | 358,164 | 309,461 |
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 | 358,164 | 309,461 |
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 | 162,798 | 153,221 |
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 | 162,798 | 153,221 |
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 | 12,883 | 13,906 |
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 | 12,883 | 13,906 |
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 | 34,425 | 31,253 |
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 | 34,425 | 31,253 |
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 |