Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investment balances
Cost	$ 50,681	$ 56,459
Gross Unrealized Gains	678	204
Gross Unrealized Losses	(35)	(178)
Investments in trading securities	51,324	56,485
U.S. Government and Government agency obligations
Investment balances
Cost	29,367	30,275
Gross Unrealized Gains	21	0
Gross Unrealized Losses	(15)	(2)
Investments in trading securities	29,373	30,273
Money market funds
Investment balances
Cost	9,736	14,127
Investments in trading securities	9,736	14,127
Equity funds
Investment balances
Cost	11,578	12,057
Gross Unrealized Gains	657	204
Gross Unrealized Losses	(20)	(176)
Investments in trading securities	$ 12,215	$ 12,085