Fair Value Measurements - Adjustments to the carrying value of investments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value Measurements
Other equity investments	$ 37,700	$ 38,700
Adjustments to the carrying value of these investments
Balance, beginning of period	22,946
Impairments and downward adjustments	(1,312)
Upward adjustments	272
Balance, end of period	$ 21,906