Investments - Summary of Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments [Roll Forward]
Beginning Balance	$ 2,745
Proceeds retained and reinvested in fund			$ 445
Cash distribution during 2017	(127)	$ 0	0
Ending Balance	2,618	2,745
Venture Capital Fund
Investments [Roll Forward]
Beginning Balance	2,745	2,195	1,000
Investment made during period		550	750
Cash distribution during 2017	(127)
Ending Balance	$ 2,618	$ 2,745	$ 2,195