Marketable Securities (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Marketable Securities [Abstract]
Investment owned, balance, shares		535,265	535,265
Economic basis per share (in dollars per share)		$ 56.05
Available-for-sale securities, amortized cost basis		$ 30,000
Fair value		$ 35,156	$ 37,918
Closing share price (in dollars per share)		$ 65.68	$ 70.84
Change in unrealized net loss on available-for-sale securities		$ 2,762	$ 5,273	$ 1,455
Investment income, dividend	$ 2,141