Investments - Credit Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Cumulative credit loss
Beginning of period	$ 13,703	$ 7,844
New credit losses	0	300
Increases to previously recognized credit losses	1,995	5,859
Losses related to securities sold or paid down during the period	(11,873)	(300)
End of period	$ 3,825	$ 13,703