Investments - Transfers in and Out of Each of the Valuation Levels of Fair Values (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Level 1 [Member]
Transfers In And Out Of Each Of Valuation Levels Of Fair Values Of Investments [Line Items]
In	$ 5,100	$ 0
Out	0	0
Net	5,100	0
Level 2 [Member]
Transfers In And Out Of Each Of Valuation Levels Of Fair Values Of Investments [Line Items]
In	0	0
Out	(5,100)	0
Net	(5,100)	0
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