Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Losses
Less than 12 months	$ 4,465	$ 343
More than 12 months	101	746
Total	4,566	1,089
Fair values
Less than 12 months	134,910	99,581
More than 12 months	6,329	58,090
Total	141,239	157,671
Municipal
Losses
Less than 12 months	2	2
More than 12 months	0	0
Total	2	2
Fair values
Less than 12 months	52	53
More than 12 months	0	0
Total	52	53
Corporate
Losses
Less than 12 months	3,879	23
More than 12 months	0	0
Total	3,879	23
Fair values
Less than 12 months	103,187	7,420
More than 12 months	0	0
Total	103,187	7,420
Foreign
Losses
Less than 12 months	584	318
More than 12 months	60	705
Total	644	1,023
Fair values
Less than 12 months	31,671	92,108
More than 12 months	4,224	55,875
Total	35,895	147,983
U.S. Treasury Bonds
Losses
Less than 12 months	0	0
More than 12 months	41	41
Total	41	41
Fair values
Less than 12 months	0	0
More than 12 months	2,105	2,215
Total	$ 2,105	$ 2,215