Goodwill and intangible assets (Analysis of carrying value and accumulated amortization of intangible assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill And Other Intangible Assets [Line Items]
Finite-lived intangible assets, gross carrying value	$ 124,423	$ 60,343
Finite-lived intangible assets, accumulated amortization	(57,180)	(48,919)
Finite-lived intangible assets, net carrying value	67,243	11,424	$ 17,090
Indefinite-lived intangible assets, gross and net carrying value	104,168	104,168	$ 104,168
Syndicate capacity
Goodwill And Other Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross and net carrying value	91,843	91,843
State licenses
Goodwill And Other Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross and net carrying value	12,325	12,325
Trade name and customer relationships
Goodwill And Other Intangible Assets [Line Items]
Finite-lived intangible assets, gross carrying value	27,757	18,189
Finite-lived intangible assets, accumulated amortization	(14,940)	(13,406)
Finite-lived intangible assets, net carrying value	12,817	4,783
Distribution network
Goodwill And Other Intangible Assets [Line Items]
Finite-lived intangible assets, gross carrying value	92,729	39,831
Finite-lived intangible assets, accumulated amortization	(40,218)	(34,468)
Finite-lived intangible assets, net carrying value	52,511	5,363
Technology
Goodwill And Other Intangible Assets [Line Items]
Finite-lived intangible assets, gross carrying value	3,937	2,323
Finite-lived intangible assets, accumulated amortization	(2,022)	(1,045)
Finite-lived intangible assets, net carrying value	$ 1,915	$ 1,278