Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Losses
Less than 12 months	$ 64	$ 4,665
More than 12 months	2,288	5,781
Total	2,352	10,446
Fair values
Less than 12 months	26,557	226,755
More than 12 months	221,413	233,026
Total	247,970	459,781
Municipal
Losses
Less than 12 months	0	91
More than 12 months	64	236
Total	64	327
Fair values
Less than 12 months	0	13,366
More than 12 months	9,099	11,645
Total	9,099	25,011
Corporate
Losses
Less than 12 months	28	4,416
More than 12 months	1,061	2,272
Total	1,089	6,688
Fair values
Less than 12 months	9,652	201,965
More than 12 months	117,827	71,044
Total	127,479	273,009
Foreign
Losses
Less than 12 months	36	158
More than 12 months	1,047	2,973
Total	1,083	3,131
Fair values
Less than 12 months	16,905	11,424
More than 12 months	88,177	137,793
Total	105,082	149,217
U.S. Treasury Bonds
Losses
Less than 12 months	0	0
More than 12 months	116	300
Total	116	300
Fair values
Less than 12 months	0	0
More than 12 months	6,310	12,544
Total	$ 6,310	$ 12,544