Fair value measurements (Details) $ in Thousands	Sep. 30, 2017 USD ($) input	Dec. 31, 2016 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value measurements	$ 679,682	$ 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	$ 679,682	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	36,279	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	643,403	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	73,355	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	73,355	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	342,760	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	342,760	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	214,567	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	214,567	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	12,721	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	12,721	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	36,279	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	36,279	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