Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Losses
Less than 12 months	$ 8,568	$ 2,025
More than 12 months	5,464	2,138
Total	14,032	4,163
Fair values
Less than 12 months	373,592	220,615
More than 12 months	135,050	56,513
Total	508,642	277,128
Municipal
Losses
Less than 12 months	314	58
More than 12 months	341	117
Total	655	175
Fair values
Less than 12 months	33,036	17,023
More than 12 months	6,808	5,784
Total	39,844	22,807
Corporate
Losses
Less than 12 months	6,862	386
More than 12 months	693	111
Total	7,555	497
Fair values
Less than 12 months	266,231	81,632
More than 12 months	15,359	4,926
Total	281,590	86,558
Foreign
Losses
Less than 12 months	1,242	1,528
More than 12 months	4,103	1,727
Total	5,345	3,255
Fair values
Less than 12 months	70,048	116,130
More than 12 months	104,795	39,031
Total	174,843	155,161
U.S. Treasury Bonds
Losses
Less than 12 months	150	53
More than 12 months	327	183
Total	477	236
Fair values
Less than 12 months	4,277	5,830
More than 12 months	8,088	6,772
Total	$ 12,365	$ 12,602