Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Losses
Less than 12 months	$ 8,168	$ 2,025
More than 12 months	4,008	2,138
Total	12,176	4,163
Fair values
Less than 12 months	369,637	220,615
More than 12 months	105,906	56,513
Total	475,543	277,128
Municipal
Losses
Less than 12 months	198	58
More than 12 months	271	117
Total	469	175
Fair values
Less than 12 months	21,849	17,023
More than 12 months	5,756	5,784
Total	27,605	22,807
Corporate
Losses
Less than 12 months	6,916	386
More than 12 months	240	111
Total	7,156	497
Fair values
Less than 12 months	268,573	81,632
More than 12 months	4,755	4,926
Total	273,328	86,558
Foreign
Losses
Less than 12 months	897	1,528
More than 12 months	3,231	1,727
Total	4,128	3,255
Fair values
Less than 12 months	73,483	116,130
More than 12 months	88,709	39,031
Total	162,192	155,161
U.S. Treasury Bonds
Losses
Less than 12 months	157	53
More than 12 months	266	183
Total	423	236
Fair values
Less than 12 months	5,732	5,830
More than 12 months	6,686	6,772
Total	$ 12,418	$ 12,602