Goodwill and other intangible assets (Analysis of carrying value and accumulated amortization of intangible assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill And Other Intangible Assets [Line Items]
Finite-lived intangible assets, gross carrying amount	$ 60,343	$ 60,343
Finite-lived intangible assets, accumulated amortization	(48,919)	(43,253)
Finite-lived intangible assets, net carrying amount	11,424	17,090	$ 22,756
Indefinite-lived intangible assets, gross and net carrying amount	104,168	104,168	$ 104,168
Syndicate capacity
Goodwill And Other Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross and net carrying amount	91,843	91,843
State licenses
Goodwill And Other Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross and net carrying amount	12,325	12,325
Trade name and customer relationships
Goodwill And Other Intangible Assets [Line Items]
Finite-lived intangible assets, gross carrying amount	18,189	18,189
Finite-lived intangible assets, accumulated amortization	(13,406)	(12,187)
Finite-lived intangible assets, net carrying amount	4,783	6,002
Distribution network
Goodwill And Other Intangible Assets [Line Items]
Finite-lived intangible assets, gross carrying amount	39,831	39,831
Finite-lived intangible assets, accumulated amortization	(34,468)	(30,485)
Finite-lived intangible assets, net carrying amount	5,363	9,346
Technology
Goodwill And Other Intangible Assets [Line Items]
Finite-lived intangible assets, gross carrying amount	2,323	2,323
Finite-lived intangible assets, accumulated amortization	(1,045)	(581)
Finite-lived intangible assets, net carrying amount	$ 1,278	$ 1,742