Investments - Credit loss impairments on fixed income securities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Balance, beginning of period	$ 54,437	$ 73,391	$ 61,595	$ 73,469
Credit loss impairment recognized in the current period on securities not previously impaired	0	811	124	12,049
Credit loss impairments previously recognized on securities which matured, paid down, prepaid or were sold during the period	(6,382)	(7,760)	(10,289)	(15,579)
Additional credit loss impairments recognized in the current period on securities previously impaired	300	911	564	2,014
Accretion of credit loss impairments previously recognized due to an increase in cash flows expected to be collected	(1,577)	(2,003)	(5,216)	(6,603)
Balance, end of period	$ 46,778	$ 65,350	$ 46,778	$ 65,350