Investments - Transfers in and Out of Each of the Valuation Levels of Fair Values (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Level 1 [Member]
Transfers In And Out Of Each Of Valuation Levels Of Fair Values Of Investments [Line Items]
In	$ 0	$ 4
Out	0	(597)
Net	0	(593)
Level 2 [Member]
Transfers In And Out Of Each Of Valuation Levels Of Fair Values Of Investments [Line Items]
In	0	597
Out	0	(4)
Net	0	593
Level 3 [Member]
Transfers In And Out Of Each Of Valuation Levels Of Fair Values Of Investments [Line Items]
In	0	0
Out	0	0
Net	$ 0	$ 0