Fair value measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	$ 608,314	$ 611,934
Equity securities	81,441	98,149
Investments in debt and equity securities	689,755	710,083
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	81,441	98,149
Investments in debt and equity securities	81,441	98,149
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Investments in debt and equity securities	608,314	611,934
Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	26,487	29,835
Municipal | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	0	0
Municipal | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	26,487	29,835
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	244,479	254,316
Corporate | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	0	0
Corporate | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	244,479	254,316
Foreign
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	306,885	299,137
Foreign | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	0	0
Foreign | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	306,885	299,137
U.S. Treasury Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	30,463	28,646
U.S. Treasury Bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	0	0
U.S. Treasury Bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities (amortized cost of $634,616 and $646,728)	$ 30,463	$ 28,646