Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Losses
Less than 12 months	$ 6,177	$ 21,218
More than 12 months	36,859	14,254
Total	43,036	35,472
Fair values
Less than 12 months	152,119	432,053
More than 12 months	424,299	146,148
Total	576,418	578,201
Municipal
Losses
Less than 12 months	292	262
More than 12 months	372	10
Total	664	272
Fair values
Less than 12 months	13,477	27,491
More than 12 months	9,800	67
Total	23,277	27,558
Corporate
Losses
Less than 12 months	284	12,935
More than 12 months	19,366	5,600
Total	19,650	18,535
Fair values
Less than 12 months	18,194	193,239
More than 12 months	209,437	44,342
Total	227,631	237,581
Foreign
Losses
Less than 12 months	4,594	7,608
More than 12 months	16,856	8,604
Total	21,450	16,212
Fair values
Less than 12 months	93,809	186,221
More than 12 months	200,359	101,294
Total	294,168	287,515
U.S. Treasury Bonds
Losses
Less than 12 months	1,007	413
More than 12 months	265	40
Total	1,272	453
Fair values
Less than 12 months	26,639	25,102
More than 12 months	4,703	445
Total	$ 31,342	$ 25,547