Fair Value Measurements - Assets and Liabilities Measured at Fair Value (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Money market and mutual funds	$ 113,147	$ 162,906
Derivative financial instruments	2,198	18,299
Total	115,345	181,205
Liabilities
Derivative financial instruments	17,817	877
Fair value of earn-out consideration		920
Total	17,817	1,797
Level 1
Assets
Money market and mutual funds	113,147	162,906
Derivative financial instruments	0	0
Total	113,147	162,906
Liabilities
Derivative financial instruments	0	0
Fair value of earn-out consideration		0
Total	0	0
Level 2
Assets
Money market and mutual funds	0	0
Derivative financial instruments	2,198	18,299
Total	2,198	18,299
Liabilities
Derivative financial instruments	17,817	877
Fair value of earn-out consideration		0
Total	17,817	877
Level 3
Assets
Money market and mutual funds	0	0
Derivative financial instruments	0	0
Total	0	0
Liabilities
Derivative financial instruments	0	0
Fair value of earn-out consideration		920
Total	$ 0	$ 920