Investments - Transfers in and Out of Each of the Valuation Levels of Fair Values (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Level 1 [Member]
Transfers In And Out Of Each Of Valuation Levels Of Fair Values Of Investments [Line Items]
In	$ 0	$ 42,328
Out	0	(597)
Net	0	41,731
Level 2 [Member]
Transfers In And Out Of Each Of Valuation Levels Of Fair Values Of Investments [Line Items]
In	0	597
Out	(4,533)	(42,328)
Net	(4,533)	(41,731)
Level 3 [Member]
Transfers In And Out Of Each Of Valuation Levels Of Fair Values Of Investments [Line Items]
In	4,533	0
Out	0	0
Net	$ 4,533	$ 0