Investments Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,499	$ 129,397
Gross unrealized gains	107	2,204
Gross unrealized losses	(27)	(816)
Investments—affiliates	1,579	130,785
Equity Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	202	23,883
Gross unrealized gains	0	2,112
Gross unrealized losses	(27)	(266)
Investments—affiliates	175	25,729
Fixed Income Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	1,297	105,514
Gross unrealized gains	107	92
Gross unrealized losses	0	(550)
Investments—affiliates	$ 1,404	$ 105,056