Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Losses
Less than 12 months	$ 343	$ 4,665
More than 12 months	746	5,781
Total	1,089	10,446
Fair values
Less than 12 months	99,581	226,755
More than 12 months	58,090	233,026
Total	157,671	459,781
Municipal
Losses
Less than 12 months	2	91
More than 12 months	0	236
Total	2	327
Fair values
Less than 12 months	53	13,366
More than 12 months	0	11,645
Total	53	25,011
Corporate
Losses
Less than 12 months	23	4,416
More than 12 months	0	2,272
Total	23	6,688
Fair values
Less than 12 months	7,420	201,965
More than 12 months	0	71,044
Total	7,420	273,009
Foreign
Losses
Less than 12 months	318	158
More than 12 months	705	2,973
Total	1,023	3,131
Fair values
Less than 12 months	92,108	11,424
More than 12 months	55,875	137,793
Total	147,983	149,217
U.S. Treasury Bonds
Losses
Less than 12 months	0	0
More than 12 months	41	300
Total	41	300
Fair values
Less than 12 months	0	0
More than 12 months	2,215	12,544
Total	$ 2,215	$ 12,544