Investment Information - Net impairment losses recognized in earnings (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	$ (470)	$ (1,730)	$ (632)	$ (3,537)
Fixed maturities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	(470)	(1,666)	(632)	(3,184)
Fixed maturities | Mortgage backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	(81)	(92)	(123)	(1,411)
Fixed maturities | Corporate bonds
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	(241)	(1,401)	(361)	(1,402)
Fixed maturities | Non-US government securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings			0	(198)
Fixed maturities | Asset backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	(148)	0	(148)	0
Fixed maturities | Municipal bonds
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	0	(173)	0	(173)
Equity securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings			0	(186)
Other investments
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	$ 0	$ (64)	$ 0	$ (167)