Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Losses
Less than 12 months	$ 254	$ 343
More than 12 months	41	746
Total	295	1,089
Fair values
Less than 12 months	7,671	99,581
More than 12 months	1,253	58,090
Total	8,924	157,671
Municipal
Losses
Less than 12 months	1	2
More than 12 months	0	0
Total	1	2
Fair values
Less than 12 months	53	53
More than 12 months	0	0
Total	53	53
Corporate
Losses
Less than 12 months	253	23
More than 12 months	0	0
Total	253	23
Fair values
Less than 12 months	7,618	7,420
More than 12 months	0	0
Total	7,618	7,420
Foreign
Losses
Less than 12 months	0	318
More than 12 months	17	705
Total	17	1,023
Fair values
Less than 12 months	0	92,108
More than 12 months	231	55,875
Total	231	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