Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Losses
Less than 12 months	$ 146	$ 343
More than 12 months	45	746
Total	191	1,089
Fair values
Less than 12 months	16,430	99,581
More than 12 months	1,276	58,090
Total	17,706	157,671
Municipal
Losses
Less than 12 months	0	2
More than 12 months	0	0
Total	0	2
Fair values
Less than 12 months	0	53
More than 12 months	0	0
Total	0	53
Corporate
Losses
Less than 12 months	106	23
More than 12 months	0	0
Total	106	23
Fair values
Less than 12 months	13,518	7,420
More than 12 months	0	0
Total	13,518	7,420
Foreign
Losses
Less than 12 months	40	318
More than 12 months	21	705
Total	61	1,023
Fair values
Less than 12 months	2,912	92,108
More than 12 months	254	55,875
Total	3,166	147,983
U.S. Treasury Bonds
Losses
Less than 12 months	0	0
More than 12 months	24	41
Total	24	41
Fair values
Less than 12 months	0	0
More than 12 months	1,022	2,215
Total	$ 1,022	$ 2,215