Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Losses
Less than 12 months	$ 2,580	$ 4,801
More than 12 months	2,168	440
Total	4,748	5,241
Fair values
Less than 12 months	227,090	239,812
More than 12 months	57,373	5,410
Total	284,463	245,222
Municipal
Losses
Less than 12 months	58	575
More than 12 months	117	0
Total	175	575
Fair values
Less than 12 months	17,023	32,038
More than 12 months	5,784	0
Total	22,807	32,038
Corporate
Losses
Less than 12 months	386	2,189
More than 12 months	111	0
Total	497	2,189
Fair values
Less than 12 months	81,632	119,965
More than 12 months	4,926	0
Total	86,558	119,965
Foreign
Losses
Less than 12 months	1,528	1,427
More than 12 months	1,727	193
Total	3,255	1,620
Fair values
Less than 12 months	116,130	70,012
More than 12 months	39,031	3,160
Total	155,161	73,172
U.S. Treasury Bonds
Losses
Less than 12 months	53	186
More than 12 months	183	0
Total	236	186
Fair values
Less than 12 months	5,830	11,847
More than 12 months	6,772	0
Total	12,602	11,847
Equity securities
Losses
Less than 12 months	555	424
More than 12 months	30	247
Total	585	671
Fair values
Less than 12 months	6,475	5,950
More than 12 months	860	2,250
Total	$ 7,335	$ 8,200