Fair Value Measurements - Adjustments to the carrying value of investments (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Fair Value Measurements
Other equity investments	$ 42,800	$ 42,800	$ 38,700
Adjustments to the carrying value of these investments
Balance, beginning of period	26,151	22,946
Additional investments		1,411
Upward adjustments	265	3,642
Impairments and downward adjustments	(1)	(1,584)
Balance, end of period	$ 26,415	$ 26,415