Fair Value Measurements - Adjustments to the carrying value of investments (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Fair Value Measurements
Other equity investments	$ 42,200	$ 42,200	$ 38,700
Adjustments to the carrying value of these investments
Balance, beginning of period	21,906	22,946
Impairments and downward adjustments	(2)	(1,314)
Additional investments and upward adjustments	4,247	4,519
Balance, end of period	$ 26,151	$ 26,151