Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Losses
Less than 12 months	$ 4,172	$ 21,218
More than 12 months	29,887	14,254
Total	34,059	35,472
Fair values
Less than 12 months	185,242	432,053
More than 12 months	377,551	146,148
Total	562,793	578,201
Municipal
Losses
Less than 12 months	191	262
More than 12 months	134	10
Total	325	272
Fair values
Less than 12 months	20,666	27,491
More than 12 months	4,033	67
Total	24,699	27,558
Corporate
Losses
Less than 12 months	1,990	12,935
More than 12 months	15,131	5,600
Total	17,121	18,535
Fair values
Less than 12 months	50,979	193,239
More than 12 months	165,730	44,342
Total	216,709	237,581
Foreign
Losses
Less than 12 months	1,335	7,608
More than 12 months	14,541	8,604
Total	15,876	16,212
Fair values
Less than 12 months	84,849	186,221
More than 12 months	206,510	101,294
Total	291,359	287,515
U.S. Treasury Bonds
Losses
Less than 12 months	656	413
More than 12 months	81	40
Total	737	453
Fair values
Less than 12 months	28,748	25,102
More than 12 months	1,278	445
Total	$ 30,026	$ 25,547