Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Losses
Less than 12 months	$ 498	$ 4,665
More than 12 months	84	5,781
Total	582	10,446
Fair values
Less than 12 months	57,239	226,755
More than 12 months	25,568	233,026
Total	82,807	459,781
Municipal
Losses
Less than 12 months	0	91
More than 12 months	1	236
Total	1	327
Fair values
Less than 12 months	0	13,366
More than 12 months	560	11,645
Total	560	25,011
Corporate
Losses
Less than 12 months	1	4,416
More than 12 months	0	2,272
Total	1	6,688
Fair values
Less than 12 months	992	201,965
More than 12 months	0	71,044
Total	992	273,009
Foreign
Losses
Less than 12 months	457	158
More than 12 months	83	2,973
Total	540	3,131
Fair values
Less than 12 months	53,913	11,424
More than 12 months	25,008	137,793
Total	78,921	149,217
U.S. Treasury Bonds
Losses
Less than 12 months	40	0
More than 12 months	0	300
Total	40	300
Fair values
Less than 12 months	2,334	0
More than 12 months	0	12,544
Total	$ 2,334	$ 12,544