Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Losses
Less than 12 months	$ 4,668	$ 21,218
More than 12 months	23,641	14,254
Total	28,309	35,472
Fair values
Less than 12 months	176,240	432,053
More than 12 months	340,180	146,148
Total	516,420	578,201
Municipal
Losses
Less than 12 months	89	262
More than 12 months	72	10
Total	161	272
Fair values
Less than 12 months	16,879	27,491
More than 12 months	2,850	67
Total	19,729	27,558
Corporate
Losses
Less than 12 months	3,944	12,935
More than 12 months	11,476	5,600
Total	15,420	18,535
Fair values
Less than 12 months	111,914	193,239
More than 12 months	115,713	44,342
Total	227,627	237,581
Foreign
Losses
Less than 12 months	539	7,608
More than 12 months	12,018	8,604
Total	12,557	16,212
Fair values
Less than 12 months	33,353	186,221
More than 12 months	220,333	101,294
Total	253,686	287,515
U.S. Treasury Bonds
Losses
Less than 12 months	96	413
More than 12 months	75	40
Total	171	453
Fair values
Less than 12 months	14,094	25,102
More than 12 months	1,284	445
Total	$ 15,378	$ 25,547