Investments - Credit loss impairments on fixed income securities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Balance, beginning of period	$ 58,628	$ 76,979	$ 61,595	$ 73,469
Credit loss impairment recognized in the current period on securities not previously impaired	0	668	124	11,237
Credit loss impairments previously recognized on securities which matured, paid down, prepaid or were sold during the period	(3,071)	(2,938)	(3,907)	(7,818)
Additional credit loss impairments recognized in the current period on securities previously impaired	162	873	264	1,103
Accretion of credit loss impairments previously recognized due to an increase in cash flows expected to be collected	(1,282)	(2,191)	(3,639)	(4,600)
Balance, end of period	$ 54,437	$ 73,391	$ 54,437	$ 73,391